Constantly being attacked

Humphrey is always getting attacked by my neighbours cat. The other cat is a girl and has apparently been done. She is the one who bit Humphrey last year and gave him a nasty abscess and cost me £400 in vet fees!! (hence he insured how lol!) Humphrey has been neutered

The owners told me she is vicious to all the other cats in the neighbourhood as well. They are really apologetic but say they dont know what to do. She comes through his cat flap just to attack him and pee in my house!

Bless him just rescued him again now, although I do try and let him see her off himself. He manages to growl at her through his cat flap, makes a noise at her then legs it! As soon as she hears or sees us she legs it as well.

Any ideas on how to deter her?

I read that putting used cat litter around the perimeter of an area you do not want strange cats to go in will keep them away. I have tried it, but honestly, we really did not have a cat problem, I was nipping one in the bud. I have had no strange cats in my yard, though. I did it because Pepper got so excited at seeing a cat outside, she almost broke the window.

Originally Posted by claire1973

The owners told me she is vicious to all the other cats in the neighbourhood as well. They are really apologetic but say they dont know what to do. She comes through his cat flap just to attack him and pee in my house!

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Next time she gets into your house trap her and return her to the owners with a formal complaint. I am not familiar with your animal laws but I think you can demand some kind of compensation for damages done to your property and to Humphrey.
My cats go outside and they are the ones keeping strange cats off the property. Once in a while they need my assistance. I usually just clap, make a noisy shhhhh and say GO HOME. Doing it two or three times in a row (may take 3 consecutive days) will stop them from visiting again.