If Frank Field were a vengeful man, then he would have cause for celebration. A decade after a brutal clash with Gordon Brown over welfare reform led to his abrupt removal from high office, the 65-year-old Field successfully championed a backbench tax revolt that forced his old foe – now Prime Minister – into abject retreat, if not humiliation.
To make matters even more piquant, Field began the week with a radio broadcast in which he accused Brown of "tempers of an indescribable nature", and stating that Brown was "so unhappy in himself" as Prime Minister that "I would say to him, talk to the people who you love most and who love you, and act on their advice". By which, of course, the Labour MP for Birkenhead meant that the leader of his party should resign and let someone else be Prime Minister.
That, however, was before the current Chancellor rewrote Gordon Brown's last Budget, and came up with an unprecedentedly improvised £2.7bn adjustment in personal tax allowances. An evidently surprised Field responded in the House of Commons with a public apology for his remarks about Brown: "I allowed my campaign to become personal. I much regret that."
When I saw Frank Field in his parliamentary office at the end of the week, it was clear that this senior lay member of the General Synod of the Church of England (and stalwart of the Prayer Book Society) still felt deeply troubled by the savage tone of his remarks, rather than any doubt about their essential honesty.
"When I heard what I had said, I was shocked. And that's what I wanted to apologise for. I found my behaviour shocking. I am criticising myself for my behaviour, not my honesty."
I said I found the phrase "when I had heard what I had said" rather bizarre. Didn't he know what he was doing or saying?
"It's one of my many failings ... I feel ill before I make any speech or intervention and I don't get down to thinking about what I'm going to say."
I suggested that this expressly did not mean he hadn't told the truth as he saw it, and that his remarks about Brown's psychological unfitness for the office he now holds were all the more devastating because they seemed so transparently honest.
"All I'm saying is that it was unacceptable to speak like that. I would have been horrified if Gordon had spoken about me in such terms and I was horrified that I had spoken about him in those terms. It's a question of proper behaviour, of being civilised."
And what does Gordon think about it all?
"Actually, he has been very generous about it. He phoned me up [after the apology] and thanked me. So that doesn't fit into your little box about how Gordon behaves. Actually, I was surprised. I wouldn't have thought that he would do that – and I didn't think we'd get the statement we did from the Chancellor, so on both counts I have been proved wrong."
But was Field bullied by the whips or his parliamentary colleagues into apologising to Brown on the floor of the House?
"No, they know me much better than to try that. I can honestly say I was not put under any pressure." I ask to see Frank's fingernails to see if any have been pulled out. "No, look, they're still all there," he giggles (and Frank Field is a great giggler, despite his austere demeanour and reputation for high seriousness in all matters).
Nevertheless, I say, some of Brown's closest allies, such as Ed Balls, had publicly attacked him over his remarks and his motives. Field recalls that when he had been battling with Brown over welfare reform 10 years ago, some of the same people had maligned him with off-the-record briefings: "So there has been an improvement in their character as far as that goes, which we should be pleased about. If they say that I have fought this campaign because I am bitter about what happened 10 years ago, then it says more about them than it does about me. I was shattered at the time, I admit, but – unless I am kidding myself – I have moved on."
Frank Field remains mystified as to how things had degenerated to this point. When Gordon Brown removed the 10p tax band in his final Budget, Field was just about the only MP who sought a meeting with the then Chancellor to challenge him on it – and he does not deny that this was one of the occasions when he witnessed a fearsome Brown tantrum. On a psychological basis, one can understand the rage: Brown has always measured his own morality by how much he does for the least well-off, and here was Frank Field, former head of the Child Poverty Action Group, author of almost 50 pamphlets on such matters, telling him he was betraying the poorest in the land.
"I'm genuinely puzzled that when one of the great characteristics of Gordon Brown's life is a burning desire to shift resources to the poor, that he ended up in a position, despite his record, of harming them. I can't explain it. It's a question you should address to him. He genuinely seemed to believe that his tax credits would sort the matter out and that I was wrong."
There's a bit of history here, too. One of the reasons why Frank Field and Gordon Brown fell out in the first year of the Labour government was that Field believed that the tax credits were a huge incentive for corruption and fraud – he wanted an insurance-based benefits scheme, rather than a means-tested one.
Not surprisingly, Field takes quiet pleasure in the fact that Brown's proposal last week for the financing of care for the elderly rests on exactly such an insurance-based scheme.
"Gordon has not taken such a view in the past, and I welcome it because it aligns him much more closely with working-class morality, which is that we should all pay some sort of entrance fee before we get benefits."
Would that be Tory working-class morality? Frank Field is the proud son of such people and indeed joined the Young Conservatives "before I was shoe-horned out of the party because I began handing out anti-apartheid leaflets". Field insists that his views, including a profound Euroscepticism, dismay at the extent of recent Eastern European immigration and a strong sense of the distinction between the deserving and the undeserving poor, are not left or right, but just in tune with the views of his working-class constituents in Birkenhead.
More provocatively (at least to some of his Labour colleagues), Field says that Margaret Thatcher "is certainly a hero. I still see Mrs T from time to time – I always call her 'Mrs T', when I talk to her. The last time I saw her I asked her what her biggest disappointment was, and she said that despite her tax-cutting strategy, the fact that there hadn't been a huge renaissance in charitable giving – I told her that perhaps on the basis of that comment we should draw up acceptable behaviour contracts for the rich!"
That is a very New Labour idea – satirically so – but I couldn't help wondering if David Cameron's resurgent Conservatives had recently attempted to suborn Field, to get him to cross over. Field seems to suppress another giggle.
"They have occasionally been very ... obliging is, I think, the right phrase. But I'm not shifting. I fit, historically, very easily within the Labour Party."
In truth, Field would be an uncontrollable maverick in any party, perhaps in part because of his personal circumstances. He has no children, no partner, no "significant other". In other words, he has the professional independence that comes with having no dependents.
"Yes, it makes a mega, mega difference" Field agrees. It does give me a lot of freedom."
It was notable that during the parliamentary debate in January on the suspension of Derek Conway for abuse of his MP's perks, Field was the most brutally critical, using the word "embezzlement". Did he not think that some of his parliamentary colleagues might be thinking that "it's all very well for Frank to get on his high horse – he's never had to worry about school fees, or paying the mortgage on a big family house"?
"I would hold my hands up and say that they are absolutely right! It's a fair criticism to make of me."
With similar self-effacement, Field refuses to accept the Government's climb-down over Brown's last Budget as a personal triumph.
"I didn't win. The real reason this change occurred was that the Parliamentary Labour Party was determined. I had never seen anything like it before. I just happened to be near the front of all this. So I could have ranted and raved and waved my arms and legs about for years, which would have had no effect at all. It was the Parliamentary Labour Party that shifted. That's what made the change.
"Also, this is fascinating politically for another reason. I see this as the emergence of Alistair Darling as Chancellor in his own right. It was very much a package that Alistair put together, I believe, and the chemistry of the Cabinet will now be stronger for that; and Gordon will be a better Prime Minister as a result of that."
Perhaps another reason for Field not to sound too triumphant is that "Alistair's package" was in fact not specifically tailored to the "10p" losers, but was a lifting of personal tax allowances overall. The biggest beneficiaries will be basic-rate taxpayers who had already gained from Brown's last Budget. Those on the very lowest incomes – for example on £8,000 a year – will still be a little worse off than they would have been if the 10p band had not been abandoned.
"I'm not claiming this is a victory at all. I'm jolly pleased with what the Government has done, but they clearly still have an operation to do more for the very lowest paid and I'm confident that this will be picked up."
And with perhaps the faintest air of menace, Field adds: "When you've just gone through a terrible shock, as this Government has, then you don't want to go through something like that again in a hurry, so I think they will move heaven and earth to cover those gaps."
I wonder. The timing and extent of the concession was clearly determined by the imminence of the Crewe by-election, wasn't it, Frank?
"Well, the electors of Crewe can feel very pleased. They have had their desired impact without throwing out the Labour candidate. Without the package, it would have been another story."
It is, of course, the grossest abuse of political convention to announce a tax hand-out in the middle of a by-election campaign – there has been nothing like it since Barbara Castle promised the building of the (grotesquely uneconomic) Humber Bridge during a Hull by-election in 1966.
"Well, unlike the Humber Bridge, this will be used and spent rather quickly. Anyway, you can't have it both ways: you in the media can't say we don't respond to public concerns and then say how shocking – they've done it just before a by-election." We can, actually, Frank, I say – and he gives another giggle.
There has always been an indefinable lightness of being about Frank Field – hard to describe unless you've met him: it's as if, like a certain sort of Catholic priest, he has never been weighed down by the worldly concerns that afflict the rest of us. He assures me that at the end of such a week, this is not exactly how he feels.
"I don't feel a particular sense of exhilaration. I feel tired, that's all. I long for a long sleep." There may be a few people around Gordon Brown who wish that Frank Field doesn't wake up for another couple of years, at least, I suggest.
"Yes, perhaps they do."