Neil,
it seems that you're hit by the problem which has already been mentioned
here.
I'm sorry but I don't know about a solution or workaround. You could try to uninstall the game, re-install it from scratch, apply the 1.2 patch and start level 15 again from a sane savegame. However, I don't know if that will help.
I've got the savegames for download
here.