Well I'm very pleased to report that I'm not squished, even though I did end up crawling underneath
I've decided not to believe anything my Dad tells me - he was the one who did the measurements. I've measured the suspension myself, peered at it very closely, and I can only find 1cm difference and no obvious signs of problems. The rear bumper is bent by about 3cm though (no idea how that happened) so that makes it look worse than it is. My housemate arrived home as I was wandering round with a tape measure and my Haynes manual so he measured it as well, and got the same numbers.
There's certainly nothing wrong with the chassis, it's not even showing any surface rust and is remarkably solid for a car that's only ever had a sill replaced.
I think Dad was just trying to scare me so he can keep my car.... he'd been looking after her while I was on holiday as he doesn't have a Mog of his own at the moment
