Got some guards to install (or ask the fitter to do so when they have the bumper off for my towbar). Couldn't figure out one point though - does the flap go over the wheel arch lining felt (ie next to the screw head) or under it (i.e. next to the bumper plastic)?

Over.

Over. The miiddle screw may be too short to catch the threaded insert when the flap is on.

I had this problem and am currently awaiting some longer screws to turn up. If they prove successful then I'd be happy to mail you a couple FOC as I had to order a pack of 100. I'll let you know how they work.