Finally some good weather so the car's had a good clean and I got round to fitting the mudflaps. Here's a few pics/tips:
1. Clean the inside of the wheel arch
2. Fit the plastic clips to the mudflap, prop a measuring tape up against the bodywork so you can use both hands to position it and get the right height.
3. I set mine at about 5cm; much lower than that and the lower screw runs out of plastic lining to screw into. 5cm puts it a cm or so above the bottom where the lining is just a bit more rigid.
4. Once positioned start screwing the mudflap in place, but just start the hole don't screw all the way through yet (to maximise the grip: straight through plastic it'll be strongest the first time you screw it in and get weaker each time you remove and replace the screw)
: use pan head screws as shown; you want the flat edge (along with a washer) to sit flush on the mudflap and evenly spread the torque. This'll stop the hole in the mudflap wanting to widen and come away under the load of wind buffeting.
5. Take the flap back off and I put a good squidge of sikaflex 252 (or no more nails or similar if you like) on the part of the flap that'll be in contact with the plastic lining. See my previous post above for why I used Sikaflex. Anyone near me is welcome to come borrow it if they want.
6. Just to be doubly sure in case I slipped, I put masking tape around the wheel arch to make sure I didn't get Sikaflex on the paintwork. Sods law says if I didn't I would slip!
7. Re-fit the flap and tighten the screws. Don't keep turning the screw once it's down or again, you'll thread the hole in the lining and lose the grip of the screw.
8. Lastly, I wiped down the flaps with a little bit of WD40 on some kitchen roll to give them a nice sheen.
9. Stand back and admire!