The ones Halfords do are really good, you don't even need thick ones! I've used them on many cars, even curvy bumpered ones and they last forever!

My best mate was having trouble fixing his plate onto his VX220, I suggested these ones and it hasn't moved since :)
 
Halfords do thicker sticky pads to cope with curved surfaces. They do work.

Not on my Leon they didn't. Although I suspect the March weather had a factor in it when the garage applied them. They had used their own pads and the rear one fell off so I went and bought the Halfords ones and gave them to the garage to use. Picked the car up on the 1st of March and the rear one was coming undone by the 2nd.

Mine are screwed on at the mo.
 
you can get a bracket with a sticky pad on it. you still have to screw it to the bumper but they are unseen and you dont have a bad looking screw through the number plate