Sadly, there's not been that much improvement since I posted this. What I have done is buy a harness & lead for Pomme. During the evening, I have them in the same room, with me holding Pomme's lead. That way she can prowl around, or sit on my lap, but can't get to Pumpkin. She does try to lunge at Pumpkin sometimes, but we spray her with water to try to discourage this. She is lunging less now & spends much of the evening on my lap.
I give them their supper in two dishes (they eat from either) and offer them bits of chicken or ham as well throughout the evening.
Pumpkin is a bit wary - she hides under the table for most of the evening, but does come out for food. She is becoming a little more confident, and comes out occassionally. Pumpkin takes up "subservient" mode (ears back, low to the ground) when moving round, and growls/hisses if Pomme gets too close.
For the rest of the day, either Pomme or Pumpkin is in our room, with their own litter tray & food, while the other roams the house. Pomme tries to get in when she's out, and out when she's in; Pumpkin tries to get out when she's in, but isn't bothered about getting in. I still think there may be a fight if they meet - mostly instigated by Pomme - so I'm concerned about not letting them meet except under controlled conditions.
We've been doing this for two weeks now. My question is: when will I know it's OK to let Pomme off the lead? I would hate to do it too early and undo the good we've done.
Does it sound like we're going the right way? On another forum I got a sound telling off because I said that Pumpkin defacates when there's a fight (she does this sometimes - usually when she has loose bowels, which she does if she has too much wet food) and is "clearly petrified". But she really truly does seem like a happy cat the rest of the time - and, incidentally, seems completely unconcerned by the fight minutes afterwards...