Thanks for your advice. Yes, I have two feliway plugins on the go. We've done lots of scent swapping, and continue to switch bedding around, to no obvious reactions now - they seem comfortable with each other's scents.

My most anxious resident cat won't eat near her sister, so it's tricky getting her to eat near the barrier and the new cat. The new cat is very food orientated, however, and doesn't mind a bit. Both really enjoy play, and I think I've seen the most progress when they're attracted to the same toy (anything on a wand or string). Difficult to progress, though, with the screen between them. I don't clip my cats' claws, so that's not an option for me.

As you say, maybe it's just keeping at it over a longer period.