Thanks for the advice. The cat flap is actually magnetic, but as he hated wearing the somewhat cumbersome tag around his neck, i stupidly took it off. I have not had any problems for the past year with intruder cats, but now this larger cat has started to come in I have lost my own. If I get him back ever, then the tag will go on for sure. lesson learnt I guess, but thanks for the reply anyway. Jo