we use some really good double side tape at work for joining reels of poythene ,i used it to put plate back on the front of the other halfs car and hasnt come off yet ,if you get stuck could always sling some in the post for you mate
Can you not do the old super magnet trick with the number plate? means at shows it can be removed and leave no marks...saw it over on the mk5gti.co.uk site. Not sure it can be done with the mk3 but woulda thought so. magnet has to be strong enough to "pull" through the plastic of the bumper