My first reaction on hearing of Margaret Thatcher’s death in 2013 was: ‘Great — now my autograph from her will go up in value.’ This wasn’t callous. It was a simple application of demand and supply. As a child of the 1980s I had learned my lesson well. The Lady wouldn’t have objected to me viewing her signature as a pension plan. Indeed, it’s what she would have wanted.

How many Caribbean villas, then, should I be thinking of buying? Because this is no ordinary autograph. I asked Mrs T. (as she then still was) to write out ‘There is no such thing as society’ and sign it. She embellished the quote, adding ‘…there are only individuals, it is we who bear the responsibility’. Thatcher memorabilia, as you’d expect with the most famous politician of her age, can be valuable. One of her Spitting Image models sold for £5,000, while a handbag fetched £25,000. ‘Ah,’ says Matthew Haley of Bonhams, ‘but there are fewer of those than there are of her autographs.’ Signed copies of her memoirs go for only a couple of hundred quid. Even a picture of her with the Queen, Ted Heath, Jim Callaghan, John Major and Tony Blair, signed by all the PMs, only achieved £1,500.

The really big money, explains Haley, lies over the pond. ‘There’s a certain reverence about US presidents that you don’t get with prime ministers. Apart from Churchill, of course.’ A bust of Winston (for which the sculptor had to endure a 2 a.m. sitting) recently made £13,000. One of his half-finished cigars went for $4,500. Compare that with Abraham Lincoln’s letter to a schoolchild: yours for $60,000.

‘There are also important state documents, about events that have changed history,’ adds Haley. ‘The Brits hang on to those, whereas in the US you do get them coming on to the market.’ Hence a George Washington letter about the US constitution selling at Christie’s for $3.2 million. Though even Stateside prices can sometimes be lower than you’d expect. A handwritten Ronald Reagan letter about capital punishment is on sale for $2,500. A Gerald Ford ‘congratulations on your Bar Mitzvah’ card will set you back just $18.95.

The politicos who buy memorabilia have varying tastes. After the Labour MP Tony Banks died, his collection of portraits, busts and cartoons realised serious money at auction, several of the items reaching five figures. (How could he have afforded them? He can’t have put it all on expenses.) Jonathan Isaby, chief executive of the Taxpayers’ Alliance, prefers leaflets and mugs. He also owns a William Hague baseball cap. ‘He’d just worn one on the legendary log-flume ride, and given how badly that had gone I couldn’t believe the Tories were still selling them. I got it for a fiver, I think.’ The front says ‘Hague’, the back promises ‘A fresh future’.

So if my Thatcher autograph is only worth a few hundred pounds, I’ll probably hang on to it. What you really want, friends in publishing tell me, is the right copy of Ted Heath’s autobiography. A rare unsigned one is worth a fortune.