Thanks, Vic - I guess the spare space-saver went missing at some point before I bought the car...
I'll have a good look at the well and work out what kind of support I could rig up. I was wondering about stuffing it with something light, like sponge wrapped in plastic, but covering the well with something rigid might be simpler.
It's odd though that the boot floor stayed up so well until the first time I meddled with it - I keep thinking I must have dislodged something that could be put back correctly, but I've not managed to work out what.