sticky pads for rear number plate

Interesting. I've never seen a sticky pad used to help hold a license plate in place. But I'm pretty sure I can imagine them and can see where they'd come in handy. What probably will work for you is Scotch Indoor/Outdoor Mounting Tape which can be found in most hardware stores, etc. The tape is close to semi-permanent in normal use (including car washes and rain storms, I bet) when properly applied, but when you want to remove it, a careful peeling of the tape will remove it fully with no evidence on the car's painted trunk or the back of the license plate that any tape was ever there.

PS--this tape is also great for semi-permanently affixing things to your vinyl dashboard, etc. Once again, careful removal leaves the dash looking like tape was never there--for use on soft vinyl dashboards and the like, a good way to pry the tape and affixed accessory from the dashboard is by careful prying with a teaspoon.

Craig

Maplin sell outdoor foam tape on a roll. £3 for 10 Metres