Thank you for your replies
I also foster with CP Feelingbroody, but not at the moment as don't want to add to the problems and far less experienced than you
And yes, Arwen is Top Kittie (and the first cat I had so used to being boss). Niblet was 6 monthish when she came in from the colony (dying from cat flu and a manky little mess). It has sadly got worse over the past few days and Arwen went for Niblet's throat and now Niblet won't come in at all (she weed on me in terror when I tried to bring her in). At the moment she has a bed in the shed and is spending her days with the ducks
and she waits in the morning on the top of their house so that I can cuddle and feed her.
I've set up the spare room with litter tray etc and am bringing her in tomorrow morning and keeping her in for a week. I'll also as Fb suggests let Arwen come into my bedroom at night as well to give her more space but also attention away from the others. I'll let the other cats into Niblet so as to not isolate from them though (and her Caesar puss will want to cuddle up with her). I'm hoping that after a week of both on zylkene and the feliway I can try a slow reintroduction again with Arwen using a cage if need be.
What is breaking my heart is Niblet is behaving how she did at the cat colony I rescued from - over clingy and crying a lot (but thankfully the stress hasn't kicked her catflu back in which is a relief). Whereas over the last 18-24 months she had become confident and not crying although still occasional cuddles
If it doesn't work Mum & Dad will take her even if they have not moved as they have her sister from the colony and a far more easy going bunch of kitbits!
Thank you all, I'll keep you updated x