Just to update this thread on how I removed the little blighters. Ok the front was pretty simple as the bottom of the plate sits lower than the plastic bumper so it was easy to just pull the bottom of the plate on one side slowly tearing away the foam pads, once it was moving some gentle pressure meant it kept on tearing away slowly.
The rear plate was a right royal pain in the arse, I could just about pull it far enough away from the body work to get a squeezed finger behind. I ended up using a wooden food spatula
on one side and again just gently easing the foam apart, there were two strips top to bottom on each end and one in the middle, nice!
Once the plate was removed I dabbed some white spirit on the remaining foam pads to loosen up the adhesive, once this had got to the sticky stuff it just peeled off the bumpers using a finger nail mostly. I reckon in total it took about 3/4 of an hour just to remove 2 number plates and clean up the bodywork!