I just wanted to say a massive thank you to everyone for all your help, support and advice in relation to the issues I'm currently enduring with my two boys. I don't think we're out of the woods yet, however, I feel so much more optimistic that this will be resolved now, whereas when I first started this thread I was pretty much in the pits of despair!
Last night was pretty much a repeat of the night before with me removing Jagger from the cat tree as soon as there was an aggressive reaction from Hendrix, so I'm now casually dropping hints to the other half that another tree may be in order
He still looks horrified at the prospect though!!
Jagger came home this morning very jumpy, checking around the house etc and when he saw Hendrix his tail puffed up and he jumped down from the kitchen counter where he had anxiously been checking what was going on out of the window. He started towards Hendrix with the aggressive walk that he does to warn off other cats but Hendrix didn't move, just sat down and watched him - once they were close enough for a sniff to confirm their identities he was absolutely fine again, visibly relaxed and went off to the spare room to go on top of the wardrobe for a nap.
I did hold my breath as he checked Hendrix out but it absolutely confirms this is all to do with other cats and the goings on outside.
I feel so much better able to address the situation now as I'm very sure of the root cause and after everyone's help started to work at ways to reduce the negative interactions etc. So massive thanks and I hope at some point in the future to be able to confirm that normal relations have resumed!