Thanks wemoon. It's strange that you should post that now because this week I was talking to a good friend who has known me since I was 17. She said exactly the same things and suggested the same sort of scenario that you describe, and I agreed that anything else would go against how I have always spoken about orientation with her prior to this, which is to say that I have never made out that orientation is much of an issue - that liking someone of the same sex is anything unusual, orientation has never been something I thought I needed to reasure her about - I always just wanted her to see it as not really a talking point, so for example I never have explained that our neighbours (two men) are gay, that her great aunts are gay, that characters on tv are gay etc any more than I would explain that a couple were straight. She knows they are together as couples because she sees them together as couples and hears them spoken about in that context. I think that's another reason why I am uneasy about having a sit down chat or whatever, because that would go against that and might make her feel it was more of an issue than I've made out, I think she'd be confused by it.
The day before the chat with my friend I was talking to my daughter about us moving out and living in a different house to daddy, she's not entirely enthusiastic lol so I revisit it regularly so that she knows it's happening, the next day she asked why don't me and her dad get married! He laughed and was goading me saying "yeah lets get married!" I was saying no I don't think so and it was generally light hearted but he kept on - even when the subject changed a few times he still brought it back and asked "yeah why don't you want us to get married then?" laughing because it was encouraging her to keep winding me up (she thought it was very funny), so in the end I said to her "no me and daddy can't get married, because I don't like boys cus boys smell" (before I get any complaints, she has a favourite t'shirt with that written on the front - I don't really think that boys smell! Well not all of them). She then started jumping up and down saying "you could marry a girl instead!" I said yes I would prefer that actually, all the time carrying on with what I was doing which was cleaning the kitchen lol. She then went on to list possibles out of all our friends

And I laughed and said no I don't like any of them in a romantic way they are just friends, but maybe someone else.
I think I will just let it go on like this, I will talk him round - I'm trying to help him see that the important thing is how she will deal with this and it shouldn't all be about him. We will move out and live seperately because we don't want to be a couple anymore, I will carry on having conversations with her like the one I described and the most comfortable scenario for me I think is yours wemoon, no dishonesty but no 'sit down this is something heavy for you to think about' kind of chats either.
Thank you all so much.