Thanks all for your input! Its useful to read this black on white like this.
My bf did talk to his children and reassured them. So did I. It seems to work for a while but then the ex gets involved. The kids are very dependent on her. When they are around us every other word they utter is 'mum'. We ignore it if the topic does not concern us generally.
He also, on occasions where the kids were rude to me, put them right and set the rules quite firmly. I think that as a result of us setting clear rules of behaviour in our house the older one stopped coming. She (twice) shouted in my house, telling me it was not my house and I should leave... running chaos and creating drama... We both talked to her together after this and while showing care and understanding we made it clear that this behaviour is not acceptable. She is used to run her mother's house as her mother admires her and is scared of her. The younger one craves boundaries but it seems that whenever she is over with us she just closes up, but I know she is finding living with her mother tough lately...
Yes, he is traumatised by the divorce and yes he is scared of his kids reactions but he is very devoted to me and our relationship.
After years of trying to sort things out for everybody and be the negotiator (Vickie

I stopped about 6 months ago. So I am not trying any more to establish anything with his kids. If she does not want to come to my house - that is ok with me. If the little one wants to lock herself in a bedroom - that is OK with me too. It was hard for me at first as it goes against my principles (I think its awful for a teenager to keep in her bedroom the whole weekend apart from meal times) but I cannot win this so I gave up trying. It is upto my boyfriend alot of time but even he cant do much. He talked to the kids so many times and we both said to the girl that we would really like her to join us in watching a film or playing a game or doing the xmas decorations with me - but we always get ignored. Thinking about it - it got worse after last xmas. The girls were here and because their dad wanted to involve them in our engagement he gave me the ring on xmas eve by the tree with them. I can only imagine this angered his ex - as there on the kids relationship with me deteriorated. Whenever we see the ex (dropping kids off, at a funeral, at the school, etc) she looks at me, looks down and speaks quitely to my bf - it is so pathetic. So yes - the ex is behind all this.
There is very little we can do because the kids spend most of their lives there. But yes - I guess my fiance should be more assertive with them about the wedding.
When I brought the suggestion that they will be junior bridesmaids (some months back) - I was told that I am putting them 'second best'... I cant win...
I know that most of their behaviour stems from fear (that the ex is planting in them and that maybe my fiance is not working enough to manage) but I also think that the ex makes the kids believe that there is a possibility of them being a family (if I was not in the picture she would have been quite happy to live in a different house with the kids but still run his life - its so sick and crazy).
As for my own feelings, Junerain - it really angers me and frustrates me. I am not one to go off on one though... I tend to deal with my emotions and think of a solution calmly. It also makes me sad of course that a wedding date is not set and I am angry with him for that.
We agreed that we will start planning the wedding after we move house and sort out some work issues he has at the moment, and I am sure that we start the planning again they will start running mad... So I am thinking now just to go to register with him in a small wedding, as I dont want the drama