It turns out that 25mm thick Kingspan PU foam, favourite thickness for door panels, warps Towards the surface that gets carved. I've never noticed that with the thicker sections I've used previously - never had to make such thin, flat surfaces. I've taken about 10mm off one side of the door panels and they've warped about 25mm over their 500mm depth.
What do to? Being Engish, I've used string. With wood screws as anchors in the top and bottom edges of the PU panel, laced across the bodywork with string to pull eveything nicely back into place.
Next step is a decent layer of epoxy filler, made up of the usual Sicomin resin, mixed into Glascell 10 (AKA Micro-Balloon, 'Millions of tiny glass bubbles' etc.) This goes off to something similar to chopped strand mat in resin, at least an order of magnitude stiffer than PU foam. That should work, I'll find out whan I take the string off.