Dot-4 brake fluid.
Worked great for me.
There are some good videos on YouTube on how to do it, but basically I wrapped up the whole bodyshell in kitchen towels (Bounty!) and make sure the bodyshell is well wrapped in them and good and padded, and all parts and corners of the shell are in contact with the towels.
Then put the whole thing in a plastic bag (with no holes in it!)
And pour in a good helping of Dot-4.
Leave it for about 24 hours and the paint should rub off quite easily.
Stubborn areas (like the corners and recesses) I had to finish off with a cloth and some neat Dot-4 and just rub it till clean.
ps - wear some gloves if you value your skin!!
pps - i think the brake fluid affects the flexibility of the lexan, as afterwards the bodyshell became more brittle than it had been.
Good luck