Thank you onemorecupofcoffee for sharing your experience. It is certainly food for thought and some good points in there! I find it really useful to hear what other people have secured/negotiated. It's never as simple as straight down the middle! Thanks as well Tinkly for the advice. My solicitor has mentioned a lot of what you have both talked about, so that's reassuring too.
It's always interesting to hear from the children this has happened to, so thanks for your post scaryteacher. It's good to get some idea of how my own kids might view things in the future and to compare this to how other people have felt. STBXH seemed to think that because the children were older that they would be able to cope with it all in an adult manner. Only one of the children (DD2) has contact with him and at the time he said that she was the only one "behaving like a grown-up in all this". Arrogant arse that he is! I think he really believes that only little children will be affected - interestingly enough the OW's parents divorced acrimoniously when she was about 8 or 9, he said. So, he feels sorry for her but because his own kids are older, then it won't be so bad for them! I guess the DCs may have a relationship with him at some stage, but I think it will be imperfect and they will never really forget what he did.
So, that's the first mediation session over. The next one will be in a couple of months once further information has been obtained. The first session was really just a bit of fact-finding and initial "scene setting". I didn't have to see him and arrived before him and left after him. It was still a horrible feeling knowing that he was in the same building and chewing over what he would be prepared to give/not give. I just can't get my head round the fact that only 4 months ago, everything was joint and shared and now it's all negotiation and division. I took a good friend with me for support and company, but still managed to cry quite a bit! I was very nervous beforehand and although the session was OK and I didn't have to see or hear him, I still felt very low and quite wretched afterwards.
He sent a verbal message via the mediator to say that he wanted to reassure me that he intended to be fair and supportive to me and our children. It's in the notes of meeting as well. That pissed me off more than anything! The law will see to it that it's fair and it's a bit late to be supportive now .... I made no response; didn't trust myself 
It just felt so patronising. Gahhhh.
So, no change for the time being and things will continue as they are until, at least, the kids finish college.
Oh, it's so hard at the moment, though. I really resent the fact that he is putting me through what he couldn't face going through himself - ie, facing the whole splitting and divorce process on his own. He made sure he has somebody there, 24/7, to provide all the support, comfort, reassurance and advice whenever it's needed. I know I have fantastic friends but they do have their own lives and families and can't be there when I'm visited by the Dementors at 3.00am or wake up crying after a strange dream. I know I have the kids at home, but I don't and can't talk to them about all of my miseries; it's not fair on them and they don't want to talk about it anyway. Then I think that maybe he's not that bothered or upset anyway and is just wanting to get the details over and done with.
In true bitter fashion, I want him to experience some of what I am feeling. The grief, the fear, the uncertainty, the loss, the scary future. And the only way he will feel that is if he and his OW implode. That'll serve the bugger right. Not a serene or reasonable bone in my body, eh?
All I can do is just keep plodding on ... or buggering on as somebody else would say. I've had a reasonable few days where it's felt a bit calmer and not quite as scary and frantic. I was prescribed ADs a few weeks ago and I'm wondering if maybe they're now beginning to kick in. I've also been talking (obsessively) in counselling and I think that perhaps things are beginning to sink in and I'm slowly starting to shed some of the self-blame and guilt. One particularly useful session was when we went through his personality, past actions, behaviour patterns etc and came to the conclusion that it wasn't just me and that nobody could really make him happy because he is the sort of person who sees happiness as being dependent on external things (and people), but these never quite live up to his expectations. Well, whether that's the case or not, it certainly helped me to think along more positive lines.
As always, thank you to everybody for all your wonderful advice and support. You are all 
On a different note, I wonder if it would be an idea to have a smiley to represent bastarding, cheating, lying STBXH's? Just on Relationships board. Oooooh, I can think of some wonderful images, but would probably just get me banned.