cat question!

Can you get one of those collars with a magnet or something on it so that only that cat can get in the cat flap? I cant remember what its called but our friends used to have one because they had a problem with other people's cats coming into their house.

Not really sure what else to suggest really. I suspect she's just marking her territory and warning him off it, iyswim. We've just moved and next door have a white cat who is constantly in our garden (he uses it as a walk through to next doors through a hole in the fence!) and our cat has already had a bit of a thing with him, spitting etc, but Im sure they'll get over it, its just new thats all.

Ah magnet thing is a good idea...
thing is its been going on a couple of months now...reckon it takes longer?

Sounds like she is just letting him know who's house it is. Cats need to establish their hierarchy and will fight until this is sorted. Once sorted, they may well get along fine, but you wouldn't want her to get too upset by her territory being threatened as she might start spraying in the house.

I would go with the magnet thing on the catflap. Our cat likes to play with the other cats outside but likes to feel safe in his own house and doesn't like them coming in.