Thanks for your kind advice. But that cat won't try entering our house again, he was so scared. All our cats will be inside/outside cats soon, so we need to keep this cat flap.

Although if I think another cat might enter the house, I'll buy a electronic cat flap. (This only lets cats with a 'key' on their collar enter/leave)

In England, all cats are allowed outside (barring medical reasons/ show cats). It is considered cruel to have an inside only cat.
People in England on the whole feel, very strongly that cats are cats. They should have their own claws and be allowed out to hunt and play.
(I do realise there is a totally different attitude in USA)

The only reason Milo & Milly are inside at the moment is because they need to bond with us and finish all injections. Milly will be allowed out on Thursday and two weeks later Milo will be allowed outside too.

Our cats are strays which are used to being outside.

George used to try and unlock the windows to escape, when we first got him, he was very unhappy until he was allowed outside.

Milly & Milo currently headbut the cat flap trying to get outside and try to dive out of any external door we open. Milo in particular hates being indoors. He sits at the front door crying to be let out.

I know the general opinion on Pet Talk is inside only but this is not the norm for English cats.

PS. We do love our cats very much inside & outside !!!