I have a serious problem with my cat and need advice.

We live in a housing complex and he has been annoying the one neighbour - so she wrote a letter to the trustees and we have to keep him in our garden or get rid of him. She's the only person in the complex that doesnt love him and he goes straight to her house - its like she has catnip on her roof. We put up a palisade fence (5cm gaps and 2m high) and he climbs it.

So we are making a plan to try fix it - but he screams. ALL THE TIME. I'm going mad. My mom suggested an animal behaviourist, I dont know anyone who's used one before and it's a lot of money (as my fiance says: to spend on something with no guarantee of results). So I was hoping someone could tell me if this is a problem that can be solved with a behaviourist - we are running out of other options. Also, we just bought this house, so we can't move because that would be my first choice.

He was neutered when he was 6 months old, he's 7 now. He's a rescue cat so he has a bit of a difficult personality. He's been wearing a pheromone collar for two months - it does nothing. We have two litter boxes, he HATES using them but he also wont go in the garden with anyone watching. He has access to valerian and he has a huge climbing castle/scratching post. He gets along fine with my other cat and boston terrier.

Last night he started screaming at 10:30 and only stopped at 1am and then started again at 4am. I cant do this anymore. I work from home, and I havent been able to sleep or concentrate in more than 3 weeks. I'm taking him to the vet this afternoon to see what he suggest - but like I say, we are running out of options. Any help/advice would be greatly appreciated.