Not my cat

[Bestcatmum]

I have two lovely cats who love each other but are very timid.
I woke up this morning with a cat sitting on my chest, it was not my cat.
This cat comes to my house eleventy times a day and makes my cats protest piss although there is no violence.
I do not want a third cat - she belongs to my neighbour.
Tried a microchip cat flap and it just rusts, it's too damp in this county for batteries on a cat flap on the back door, they rust within a week.
I have no idea what to do.
I cannot tell my neighbour not to keep her cat in as mine go out freely but I hate the pissy smell when I get home.

Not sure how you are having such an issue with a microchip catflap, we have one with no issues, the side where the batteries go, goes on the inside?

Unfortunately if you have a flap on the door that allows all cats free access to your house, brazen neighbourhood cats will make use of it!

Could you lock the catflap and just let your own cats in and out when required?