Question is why do you need to remove it? If you're putting a private plate on, it will have to be swapped again. If you go to Halfords you can get some deep sticky pads with which you could stick the new plate directly on to the old one. Might save a bit of messing about. If you do decide to drill the old one out, can I recommend that when you fit the new one you smear some Waxoyl in the hole to avoid water penetration which could cause problems with the locking mechanism?