Nobody would have blamed Sir Alex Ferguson if he had walked away from Old Trafford two weeks ago after Manchester United lifted the European Cup for the third time on an unforgettable night in Moscow. What is there left to achieve? Ferguson has won 29 leading trophies at United and Aberdeen, but the most successful manager in British football is not ready to throw in the towel just yet.

First he has to stop Real Madrid getting their hands on Cristiano Ronaldo this summer and, if and when that mission is accomplished, the 66-year-old is planning to spend at least two more years in the dugout before putting forward the case for Carlos Queiroz, his Portuguese assistant, to replace him and become United's first foreign manager. “I'll carry on for no more than three years,” Ferguson said. “Maybe two more. I have to respect my wife because she's not a football fan, she's an Alex Ferguson fan. When our games are on TV, she doesn't watch them. She watches Teletext. Can you believe that?”

One headline that Ferguson is confident Lady Cathy Ferguson will not be reading on Teletext this summer is “Real Madrid sign Ronaldo”. The Spanish champions have made no secret of their desire to buy the Portugal forward and Ronaldo has left the door open on a move to Spain, but Ferguson is adamant that United will not sell their most prized asset.

“I'm very confident,” he said. “It's a game that Real play all the time. They've done it with David Beckham, they've done it with Ruud van Nistelrooy, so we're used to it and it's great in a way because it's a compliment to the standard of players that we have. I am more than confident that Cristiano will be here next year. We want Cristiano to stay for a long time.”

Ferguson rarely grants one-to-one audiences to journalists but the United manager was relaxed and engaging when he met Sir David Frost for his first feature-length interview for nearly a decade. Basking in the glow of United's double success, Ferguson talks candidly about how he has mellowed with age, his fascination with politics and history, his run-ins with referees, his struggles to master the art of playing the piano and his friendship with Gordon Brown.

Ferguson was born in Govan in 1941 and worked in the Clyde shipyards, where he made a name for himself as a firebrand trade unionist who was always ready to stand up for his rights. “I remember I came home once,” he said. “My mother said, 'Your granny wants to speak to you.' I said OK. My gran just lived round the corner so I went round and she said, 'Your mother thinks you're a communist.'

“I said, 'Don't be so stupid, I swear I'm not.' 'Well you'd better bloody not be, I'd never forgive you.' She went right through me, so I rode round the corner to ma. I said, 'Ma, I'm not a communist.'” Luckily for Ferguson, his mother believed him, but he has been a lifelong supporter of the Labour Party and he can imagine himself holding court in the Houses of Parliament if his first love had not been football.

“Maybe I'd have become a politician,” he said. “It's possible. I don't know if I would have enjoyed it, but just because I was in a trade union didn't mean I was a communist.”

It is not difficult to imagine Ferguson relishing being the Home Secretary in a Labour government, locking up criminals, tackling knife crime and giving as good as he got against Jeremy Paxman. For the time being, though, he has more pressing concerns. Can United retain the Champions League next season, are referees getting too fat, and is he going soft on his players?

“It's hard for me to lose my temper with the quality of players that we've got now,” he said. “Sometimes you regret losing your temper, but you are who you are. I remember Jock Stein said to me that there was nothing wrong with losing your temper for the right reasons. I think probably 70 or 80 per cent of the time it has probably been the right reasons.

“There are some times that I premeditate it to get the points across, to keep everyone's feet on the ground, but there have been times where you think you've said things in the dressing-room or reacted badly, strongly. But not so much now - I'm going back maybe 15 years. I'm a pussycat now.”

So much so that the fiery and irascible Scot even tried to teach himself how to play the piano last year. “One of my great ambitions was to play the piano so my wife bought me one for Christmas,” he said. “A boy from Govan with a piano, for goodness sake. I got the books and I thought, 'I'm going to learn', but I just don't have the time, I need a tutor. When I retire I'm going to get a tutor. I still try now a wee bit with the books, but I find it difficult - no left hand.”

Tickling the ivories and reading about Irish, Scottish and American history helps Ferguson to unwind away from the game, but it takes only one mention of the word “referee” to send him off on one of his famous rants.

“Refereeing should have improved, simply because of the decision to have full-time referees,” he said. “That was a step everyone wanted but it's certainly not improved it enough to say that it's worked. I don't think their fitness is as good as it should be. Fitness is important at the level we're playing nowadays. Some of them are fit - don't get me wrong, they're not all unfit - but some are not fit enough and it reflects badly on themselves because they are supposed to be full-time professionals.”

If Ferguson keeps his word - he famously changed his mind about retiring six years ago - he will leave Old Trafford to spend more time with his family and his piano. There will be tributes and tears, and do not rule out Sir Alex becoming Lord Ferguson of Govan, but who will be his replacement? Roy Keane? Mark Hughes? José Mourinho?

“I think it will be down to ability and how well some managers are doing at the time,” he said. “You can't discount my assistant, Carlos Queiroz, because I think he's outstanding. He's done fantastic work for the club.”

And what will he do? Disappear into the sunset or stay on at United as an ambassador - or a distracting influence in the background? “I deserve a rest,” he said. “The field will be left clear.”

Sir Alex: The Frost Interview, tonight, 8pm and 10.30pm, Sky Sports 1.