Ok... so. I decided to take back a bit of control and do what I can to get things straight between STBXH and me. Neither of us have any money for mediation and I have the bare minimum for solicitors fees, so I decided not to wait for conversations (or emails or texts) on his terms and sat him down today to have a talk.
I explained that I was upset and stressed about his suggestion his GF should meet DS, and that in my opinion it would still be far too early in September for this to happen, not to mention completely inappropriate for DS2. That in fact, I wouldn't consider it necessary for her to be around until such time as contact taking place with the children actually in her home, somewhere she has every right to be. (He chose now to admit he has already moved in with her and had been living with her for some time. I pointed out this is irrelevant as that is 3 hours away and contact most definitely will not be happening there.)
While he is seeing the DC in my home, or the home of one of his family members, there isn't a good reason on earth why she should come along as far as I can see. He seems to have accepted my point, and although he's not exactly agreed, I left it at saying IMO it wouldn't be appropriate for her to be involved (certainly not for me to meet her) until she actually moves to where we live, with him, and makes a commitment to be in his life that means she has to have a relationship with our kids. This will not happen for 12 months or more according to him. I pointed out that I harbour no desire to win him back and actually accept his right to have a relationship with her that I have no say in. But that this doesn't extend to our kids, where I feel the only adults who have a say are myself, and H. Turns out that she doesn't even want to meet DS, she just wants to come down for the weekend to see where they are both (apparently) planning to move to (no comment 
) and he is the one who thinks she'll be bored waiting around for him for a whole day if she does. I pointed out that this is what she has signed up for, and the sooner she faces reality on that front, the better.
I also let him know I'll be serving divorce papers asap citing adultery and naming her, so he should prepare her to receive her own copy of the papers and make sure she is willing to sign them and return them so that everyone is spared further expense etc. He seems very happy to accept that, so I guess I should be grateful that the divorce aspect looks simple enough :(
I feel better that I'm not running over this crap in my head any more, and at least now I can get on with sorting out the big D. It makes it easier to write the statement of arrangements for the children now that we've had this chat. I suggested to him mediation was always an option and he said he would sort it out asap - right up until I explained it cost £140 a session and he would be liable for the cost! At which point he felt we were doing a good enough job of talking between ourselves.
When you get down to the nitty gritty, it's all so mundane and sad. The facts can't be changed. So you just have to get on with it really. I'm glad I got through it without crying or raising my voice at least. :( Another one to chalk up to the bear hunt.