I would recommend the microchip catflap.

It is not fair to expect your neighbour to keep horse cat in so really the onus is on you to keep horse out of your house and to provide a safe haven for your own cats.

There are 2 types of chip flap: electrical socket powered and battery powered. They are provided by rival businesses (and I mean rival!)

You have to get your cats chipped, but that isn't such a bad idea. If they are chipped already you can test the chip number on the petaporte site to see if it is recognised.

A chip flap is expensive (£70-£110 depending on where you get it) but it is programmable in other ways than just ensuring your cat is allowed in.

Magnet/infra red flaps work from a tag on a collar, but are cheaper (I think). However collars get lost and can be a danger for the cat unless quick release.