It must be bad enough being the daughter of one of the most loathed women in Britain; it can't be nice having a dad who's been regularly portrayed in Private Eye as a goofy old gin-toper. But that wouldn't matter so much as long as you had a jolly brother or sister to share the burden.
But you don't. It turns out that your twin's not just a prat, but a prat on the make.
And it's not just the millions of pounds he's meant to have made out of brokering arms deals, there are other accusations around, like the fraud and racketeering allegations going through the Texan courts . . .
You may be twins, but you're different as chalk and cheese. He has homes in Belgravia and Dallas; you have a small west London house. He drives a Lotus; you drive a Mini Metro. He has bodyguards, a personal valet, an entree to oil-rich society from Dallas to Saudi; you're a trackie-bottom sort of person, a take-away pizza kind of girl. He's prickly, petulant, and always looks as if he's standing above an evil-smelling drain; you're open, smiling and joking, the ultimate good egg.
Funny, really, because by rights you should be the resentful one, clawing for material success to make up for a deprived childhood. For it was Mark, apparently, who was the doted-upon child.
In your mum's eyes he could never - can never - put a foot wrong. When he got lost on some crack-pot rally in the Sahara, she even cried in public. Has she ever cried over you? I wonder.
But there's a catch. It seems that the secret to your brother's success has been your mother's coat-tails - from when he modelled tennis-wear in Japan to the Cementation affair, and now possibly in brokering an arms deal. If he wasn't called Thatcher, Mark's name would be Nobody.
But you've done everything yourself. You've struggled as a journalist, doing bits on radio, writing the odd book.
Despite the fact that you could probably make a fortune from your inside story, you've never spilled a single political bean. And, unlike your brother, you always manage to be polite and likeable - even turning down Nick Broomfield, when he was making a programme about your mother, with charm. (Your brother simply refused to return calls or open the door.)
You've been quoted as saying: 'I do wish people would realise I'm just an ordinary sort of bugger.'
But actually, considering the weird family you sprang from, and considering that you were the less favoured child and yet have ended up with inner resources of the kind that money can't buy, I, for one, think you're rather exceptional.