I'm totally clueless as to what to do so would be grateful for any advice!
I have a 3yo female cat who has been housetrained the whole time we've had her (we don't have a litter tray as she goes outside in the fields behind the house). She fights with next door's cat (also female, similar age and size) but its always seemed pretty equal in who chases who etc and generally gets no worse than hissing and chasing each other with the occasional proper scrap which we've had to break up.
Recently I've noticed the neighbour's cat waiting by the cat flap or hiding in our garden ready to pounce on our cat as soon as she goes out! I've been chasing her away and have taken to keeping a water pistol by the back door but shes too fast for me! My cat seems frightened to go out these days which is such a shame because shes such an active outdoor cat. I've also caught her peeing on the kitchen floor on a few occasions, I assume because shes scared to go out or marking her territory 
I feel so sad for her, she only goes out if we're in the garden now and doesn't seem herself at all. I had her checked over by the vet and shes fine physically so I guess it must be down to next door's little shit cat ambushing her as soon as she ventures out. Any ideas anyone?