It is pushing on towards evening. Colonel Patrick Mercer MBE OBE, still — just about — MP, has risen from his seat at a restaurant in south London and in loyally modern-Conservative fashion, is about to cycle home through the London dusk. The waitress who has been serving us gives him a glistening smile and says: “So, what actually do you do, by the way?”

And Patrick says, ever straight-faced: “Well, I’m a disgraced Tory politician.” The waitress giggles flirtatiously and squeals: “Ooh . . . really?”

And Patrick says: “Yes. A disgraced, racist, Tory politician.” And the waitress says again, less enthusiastically, “Ooh.” And then, more quietly: “Ooh er.”

And with that he marches out, very upright of bearing, and climbs upon his bike and, back straight, pedals home.

Mercer was sacked 10 days ago as his party’s homeland security spokesman, after having given an interview to a Times Online journalist about life in the army, an institution in which he served for half of his 50 years, before he became an MP in 2001.

During this ill-fated and deeply regretted conversation, Mercer announced that it was not unheard of for ethnic minority soldiers to be called such things as “black bastard” — just as obese squaddies would occasionally be referred to as “fat bastards”.

Mercer said both insults were equally tantamount to bullying and quite unacceptable. He also said he’d come across plenty of black soldiers who had howled “racial prejudice” when upbraided for their indolence or uselessness.

All of this stuff was, when published, deemed either “offensive” or “racist” or both by his party bosses, despite, when you read the quotes in context, it is plainly neither. Anyway, he insists the interview was off the record and he harbours a fair amount of bitterness towards the journalist (hitherto a family friend) who none-theless gleefully wrote the whole thing up.

Within hours he had been sacked by David Cameron without being given much of a chance to explain his side of things. At the time, he said he agreed with the decision to sack him. Time, though, has perhaps ever so slightly altered this perspective.

“Well, I put my leader in a difficult position, by being an incompetent twit,” he says. “And I am always absolutely loyal. It comes from having been in the army, I suppose. But then,” he adds with a grimace, “I expect people to be loyal in return.”

They were not remotely loyal, though. Despite Mercer’s own protestations of party loyalty, there is not much doubt that he is unhappy at the manner of his departure from the front bench — and my suspicion is that we will be hearing rather more from him in the future, in a manner that may not be entirely to his leader’s liking.

Some of this may be put down to the shabby way in which he has been treated — a friend told me that they “wouldn’t treat a dog the way in which Patrick has been treated”. But equally, Mercer’s vision of Conservatism is not always in accord with that which emanates from Tory HQ these days. It is not so much that Mercer is right wing, despite the stereotype one might expect from someone with his military career, all that derring-do; he is, more or less, an old-fashioned one-nation Tory with a liberal streak a mile wide.

It is not inconceivable that he could one day cross the floor of the house — though, despite his present rancour, I wouldn’t bet on it. What he is most definitely not, though, is metropolitan. Not being properly metropolitan effectively got him sacked.

“Politicians have got to understand that people outside of London view the world differently from those who live in the capital. They think very different things. And you need the votes of the people outside London to win a general election. It is a different world out there.”

In his constituency of Newark, in rural Nottinghamshire, he says he has been “astonished” and “overwhelmed” by the support he has received in the wake of his abrupt defenestration. The local party, I’m told from elsewhere, has taken down the photographs of David Cameron from its walls. The e-mails have poured in — some 2,500, according to Mercer.

“And were any of them critical of what you said?” I ask.

“Yes. Seven of them. Actually six, because one chap e-mailed me again to apologise and retract his criticism.”

Do you think that what you said was wrong?

“No, God no, not wrong. But I phrased it clumsily, I think.”

You would stand by your assertion that calling someone a black bastard and a fat bastard are just about equal in their manifest unpleasantness?

“Yes, of course. They’re both bullying, they’re both hurtful. No real difference.”

Mercer is certainly not a racist; his record, as a colonel in the Sherwood Foresters, was of incessant and successful recruitment within the area’s black and Asian community. At one point, all five of his company sergeant-majors — recruited and promoted by Mercer — were black. Leroy Hutchinson, who served as a corporal under Mercer, said: “He never tolerated racism. Not a single one of his men would consider him a racist.”

So, it is not racism that has done for Patrick Mercer’s career. It is something altogether more damaging and corrosive to modern politics: candour. This is not the first time that he has been frustrated to the point of exasperation by punishment being meted out to people who speak what they believe to be the truth, in an unvarnished manner.

“It’s one thing I learnt from being in the army. You speak clearly and unambiguously, directly and without obfuscation. Then people understand what you mean. In politics, the reverse is true. The whole point is to obfuscate and prevaricate, to get up on your hind legs instead of stating clearly what you mean and proceeding to act.” It is now that he becomes animated, talking about the thing that truly concerns him — indeed, scares the hell out of him.

“Take security in London. Nearly two years since 7/7 and not a thing, not a single thing has been done to improve our security on the Tube. Not a thing! We are exactly where we were two years ago. And then, the other day, I received an enormous document on my desk — paid for by the taxpayer, commissioned by the government — entitled The Definition of Terrorism. A great long semantic work explaining exactly what terrorism is . . .” He throws his hands up in exasperation. “I mean, I’m sure it has its place. Somewhere. But it’s not the point. People will be killed. And we are mulling over the philosophy of what constitutes terrorism.”

He is loyal to the army, too, describing the recent court martial (and acquittal) of six soldiers accused of allowing Iraqi detainees to be abused as a “political show trial”. But his experience of internment in Northern Ire-land (where he worked in plain clothes) makes him ill-disposed towards the government’s wish to detain terrorism suspects for longer than 28 days without trial.

He is as candid about his misjudgments as he is about those policies where he feels he has been proven right (or, worse, is about to be proven horribly right). For example, he was one of the Conservative party’s most stoical supporters of the invasion of Iraq. Got that badly wrong, didn’t you, mate?

“Yes, yes, yes,” he says, head in hands. “Badly wrong. I should have listened to Hans Blix [the UN weapons inspector] when he begged me and other members of the select committee to give him just six months more. We should have done that, no doubt about it.” He insists, however, that he believed Saddam Hussein possessed weapons of mass destruction.

It is all too easy to consign Patrick Mercer to a box marked gung-ho backwoodsman (something that I suspect his leader has already done). But it is to miss the essential point, which is that Mercer is a plain-spoken maverick within a community of politicians where such qualities are punished rather than rewarded. For a man brought up within the rigid discipline of the armed forces he is refreshingly unconventional.

In 2000 he gave up his military career and asked me for a job on the BBC Radio 4 Today programme (where I was editor) — a brave and frankly ludicrous decision by someone who had never trained as a journalist (and a pretty ludicrous appointment on my part, too). But it worked; he was brilliant. He was a natural journalist, concise and sharp and possessed of a greater knowledge of military matters than all the other BBC defence correspondents put together.

He was regarded with intense suspicion by the largely liberal-tinted producers on Today — but he won them over. Using his military intelligence, he broke a string of important stories and risked his neck from beyond the front line in Kosovo.

His interviews with the programme presenters became the stuff of legend, for their clipped and wry observations. Are depleted uranium shells dangerous, Patrick? “Spoil your day,” he replied.

But beneath this parody of the stere-otypical army officer was a deep understanding of geopolitical forces and a gentle Conservative sensibility.

What will he do now, I ask? His options are many and varied. He could keep his head down and hope for political rehabilitation in a couple of years, although if I were Patrick, I wouldn’t hold my breath too long. He seems to be inimical to the current vision of Conservatism, though I cannot think of any better-equipped politician to preserve our domestic security. Or he could coalesce around him like-minded Tories, the legions of disaffected nonmetropolitans, and bide his time, waiting for the climate to change and occasionally firing heavy ordnance in the direction of his leader.

What does Conservatism mean to you, Patrick?

“Freedom of speech!” He announces, the eyes glinting.

Ah yes, that. And what else? “Trusting in the individual to make the best of himself.”

Anything else? “Having principle.” Oh dear me, principle. This is all ter-ribly old-fashioned stuff, don’t you think? So is David Cameron a man of principle, I ask him?

“He is the leader,” says Mercer. Yes, I know he’s the leader. I asked you if he had principle.

“He is the leader,” he repeats. Yes, I persist, but does he have principle?

He finishes his glass of water, smiles a little, narrows his eyes and, from the other side of the table, sticks two fingers up at me.

“He is the leader,” he says, with finality and stands to leave, which is when the pretty waitress, intrigued by something about this man, asks him what he does for a living.

Disgraced Tory politician, love. But he’ll be back.

Times Recommends

• No room for Baldrick in overstretched army
• Freecycle booms as middle class feels squeeze
• Marksman gets £5,000 over ‘serial killer’ quip