Overnight my inbox fills up with unsought mail. “Hair straighteners,” I was told yesterday, “are the new man-bag essential,” and that a “hitherto unknown Jacobean play” by Lord Edward Herbert (to me, a hitherto unknown playwright) has been found in a trunk in an attic in Powis Castle.

Both, however, were more welcome than the e-mails from two organisations that leapfrogged over each other between Thursday night and Sunday. At 28 minutes past midnight on Friday morning Nick Griffin e-mailed me and countless others to offer masochistic highlights of his Question Time catastrophe.

Twelve minutes earlier Frank Field, MP, in his incarnation as the Balanced Migration group, wrote responding, not to Griffin, but to Jack Straw’s appearance on the same programme. His question was as pithy as it was tendentious: “If the Government,” asked Mr Field, “is against capping the British population at 65 million, where will they set a cap?”

Mr Straw and the panel had been asked if Labour’s immigration policies were responsible for the rise of the BNP. As I understood the answers, the Conservative and Liberal Democrats said “yes” and Mr Straw seemed, tentatively, to think not. The question was good but the responses were dreadful, and there was something more. All of them saw immigration as a problem — there was no one to say that it had benefited us economically and enriched us culturally. Surely, I thought, it was this unchallenged and cowardly perception of the negative nature of migration, rather than migration itself, that was a part cause of the growth in votes for the BNP.

More of that later as we continue to root around in my mailbox, For Mr Field’s late-night population-cap demand was preceded by another Balanced Migration press release. (I got a third on Friday). This followed the release of the Office for National Statistics’ 2008 biennial population projection. The ONS projected a population increase of 10.4 million to 71.6 million by 2033. Of this 10.4 million the contribution of immigration, directly or in the form of new births, would be 7 million.

The Balanced Migration group (or Frank Field as it should more accurately be called) wanted to tell me that “the official forecasts mean that, if the UK’s population is to be held below 65 million, we will have to reduce net immigration from a projected 180,000 a year to zero”.

I should say here that the ONS begins every such report with the explicit warning that its projections are not forecasts, as Mr Field claims, but projections forward of recent trends. When I spoke to him yesterday Mr Field essentially dismissed this as nit-picking. I think he’s wrong.

Anyway, the press release said that “we are on course for an unsustainable and unacceptable rise in population. Over the next Parliament, at a time of public spending cuts, the Government will have to find the money to pay for one million new immigrants — a city the size of Birmingham.” Always Birmingham. Never ten Cambridges.

What Mr Field didn’t point out — because his intention is propagandist, not informative — was that the 2008 projection was a reduction (albeit small) from the 2006 one. Nor did he mention that the last actual figures, for 2008, showed net inward migration of only 118,000, far below the ONS projection. And even these numbers are likely to be substantially undercut by figures out next month for April 2008 to March 2009. Broadly the ONS assumptions have been based on an unparalleled period of growth. They don’t speculate on what might happen if things change.

It’s likely that net immigration will fall and the next set of projections will be revised, although they certainly won’t get down to Frank’s magic 65 million (at which point, I guess, the rivers run out of water, to be replaced by blood). One thing readers should know is that — contrary to tabloid imagination — those emigrating are not all white British citizens escaping the horrors of foreignisation, but are mostly former immigrants going home. But even if no one went in and no one went out, we would still get to around 65 million because of the birthrate and an ageing population.

So the forecasts (rather than the projections) are probably wrong. But even if you accept that, and further accept that there isn’t very much that’s hugely different between a country of 62 million and one of 70 million, Mr Field’s third objection to immigration has to be dealt with.

It’s about cost. All those migrant children with English as a second language (more than half in 1,338 schools, says Mr Field) cost a lot when budgets have to be squeezed — although he worried about it when budgets were blooming — and the babies need midwifery, houses and so on and on.

One day, I suppose, we will stop having these dialogues of the deaf where the Fields tell you all about how much migrants cost, and the responding Aaronovitches remind you that the economic benefits of relatively free migration are much, much greater. Remind you, too, that the idea that there are only so many jobs in the economy, and that they go either to immigrants or “indigenous” people, is utterly false. In late 2003, just before we opened our doors to the Polish plumbers, for example, there were about 700,000 unfilled vacancies in the UK.

But even if I could convince Mr Field of this, he has his last, trumping, objection, that, as he put it in an article last week (co-written with Nicholas Soames, MP), there remains the nebulous problem of “social cohesion”, that “England is being fundamentally changed”. Why? Because, he told me, before 1945, in the Victorian and Edwardian eras — a time of “secularised Christianity” — we all roughly agreed on what citizenship was, but migration and globalisation have changed all that.

It occurs to me, in all this, that net inward migration has again become the lightning conductor of people’s disgruntlement with change, with the ever more mobile and demanding world of the 21st century. It is utterly false to say that we haven’t talked about immigration. Many of our newspapers do very little but talk about it. They don’t “debate” it because their operating assumption, like Mr Field’s, is that it is bad; it overwhelms us; floods us; swamps us; it swarms in Sangatte, until it is closed, then infests “the Jungle”.

No other point of view is put, just as Mr Straw failed to put it on Thursday, as he dangled between denial and defence. So when people make the wrong connection between their fears of the modern world and the incomers, no one has slipped an e-mail into their mental inboxes to suggest that they might be wrong.

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