I don't want to jinx myself but I think we made progress last night...
I changed all the other litter trays to the new clay clumping litter this weekend as they were basically only using the one tray in the hall. Hendrix decided he preferred the clay litter too and only used a tray with the old litter if jagger refused to let him have a poo in the hall tray.
I didn't clean the hall tray though, I swapped it with the conservatory tray so jagger could smell his scent and possibly encourage him to use it out there again. I didn't really expect anything though. But this morning, there was a wee in there - yay! Pretty confident it's Jagger's as it was under the rim, he always tucks his bottom under the rim whereas Hendrix has his paws tucked under the rim so his wee is usually in the middle of the tray.
Jagger seems to be having more regular wee's now too instead of hanging on until he's literally bursting which must be better for him.
Jagger also used the litter tray cupboard last night which he's only used once before and missed the tray. The tray is fairly small in Bengal terms and it's always been hendrix's toilet before. Hendrix had a huff and puff about jagger using it but I was thrilled.
I have put privacy film out in the conservatory on the door the fox looks through so I'm hoping jagger feels less threatened out there now.
Neither of them are doing their business outside so the fox threat is obviously still an issue and they will pretty much stay in when I'm at work and not around to keep an eye on things and rescue them if need be.
I'm not getting too excited but I do feel I'm getting somewhere now, I have to clean the trays pretty much immediately they've been used or jagger won't use them. In fact they both wait for me to get home from work and there is a toilet run! Jagger will gave a wee, I'll clean it up, Hendrix will then have a wee and possibly a poo. Heaven forbid they should use the trays when I'm not there to instantly clean up after them