That's it, I've had enough & ordered a high powered water pistol.
New black tom appeared in the neighbourhood a few months back. Very vocal, yowls loudly often at night. Started coming into my back garden and coming up to the french doors in the living room. These are netted to stop mine getting out, but I've been worried enough by this bloody cat that I've reinforced the bottom bit with some wire mesh.
He started out by coming close, looking through the glass then lounging on the raised decking nearby. Then he started to come up to the door when to was open; mine went over to say hello and he hissed and growled at them through the net & mesh. Mine ignored this, just sat there & watched him "what you up to strange cat?", no response to him. He then started spraying the back garden & I caught sight of a rather impressive set of fluffy pom poms so definitely a tom.
Today the door was open & he came up, mine were upstairs on the bed. I went over there & he hissed and growled at ME. Didn't move away, just threatened me until I bashed on the mesh then sauntered off. He's also just been up to the front window (Flatcat installed) and again hissed at me before going to lie on the wall outside. He threatened me again when I went out of the front door, sprayed up the front of my house, then roamed over next door's front garden spraying as he went & occasionally looking at me as if to say "I own this".
Last straw was when I realised that the living room smelt nasty - turns out he's sprayed on the net curtain attached to the inside of the door 
(not envy), presumably when it was open today & I wasn't in the room. 
So water pistol ordered by Amazon Prime. I'm pretty tolerant of cats roaming around - next door's cat uses my extension roof, others used to sunbathe on the decking - but since he's been coming round I've only seen even the very large evil ginger & white tom a couple of times, and none of the others.