The trick is to be patient. It can be done without scratching your paintwork any more than it already has.
•The idea is to be gentle with it. Use soapy water and a soft, clean cloth to wash around the plate to start with.
•Get a hair dryer and heat the magnetic plate till it is pliable.
•Slowly lift the edge of the magnetic plate. You may need something firm to help lift it up. Something like a plastic ice-cream container lid cut in half.
•As the magnetic plate heats and becomes more pliable you can push the plastic underneath a little bit at a time.
•Try to keep it lubricated as you do that. Squeeze or spray some soapy water underneath as you go.
•Keep at it, bit by bit and hopefully it will come off without any more damage.
•Give the area a wash and polish to finish.
Never buy them again. Use p plate clips instead.