So Mr and Mrs Redhen have had a chat. 
Basically started with me telling him that I am seriously considering going back to my old house as I am so fed up with it all. Funnily enough, instead of the normal dismissal I seem to get when I try and discuss things, he actually took notice this time.
Told him I am basically fed up of being bottom of the heap, I am fed up doing loads of stuff with his kids, while my own child gets no quality time whatsover. Told him I am fed up with him telling me constantly how he wants his kids all the time, but doesn't have the decency to consult the woman who is left with them for hours on end. Quoted some things from this thread about being sub-sumed and morphing into his family. Said I was sick of not being considered, treated like I don't matter and that DS and I have given up a lot to come here and I am not f*ing Mary Poppins!! Told him I am worried about DS self esteem and confidence and that I have no time or space to work with him on it. My own child is being neglected and I am not having it anymore.
He has said that when he goes off and leaves me with the kids, it is only what he would do if I weren't here. I pointed out that I AM here and he needs to consider me. If anything happened to his children while they are in my care, I am the one who would be in trouble, not his 14 yr old who he seems to think can be in charge when he isn't around. 
I have said all I want is for him to consider my needs and those of DS too. Not just what he wants all the time.
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I honestly don't know if he "gets it". He said he has only tried to make me and DS feel included by us all doing things as a "family". When I pointed out that I spend more time with his children on my own than he spends with DS who lives with us, I think it surprised him. I also told him that my ex is being very helpful and I can now have DS on the weekends that we don't have step children here. He didn't like the thought of not getting time alone without children (or is that just my child?!)
I told him I am dreading the summer when he works 15 hr days, 7 days a week and his ex tells him to have the kids for extra time, because I said this is what I expected he would agree too, again, without consulting me. I would still be expected to look after them at the weekends at this time anyway. When I have mentioned it before, he said he would agree to it, he denied this last night, but I can't live in this sort of dread.
I did try to get him to see that if we lived in separate houses, we could still be in a relationship. I even tried to say that he could parent his children more freely without the rules I try to impose (manners, tidiness, respect etc.). Of course, he doesn't see any advantages to living apart because HIS life has just carried on as before, but now with added enhancement.
He has agreed that I need to carve out some time for my DS without his kids, albeit we haven't worked out the details yet, that he should "consider" me when he makes decisions involving the children or our home. I just don't know if I can make him really understand, to be honest. I do love him and I do think in a lot of ways he is really trying. He just can't understand why he just can't carry on as he always has.
He was very quiet last night when he thought I might be moving out, and I'll be honest, because he wasn't actually following me round the house, or hanging over me when I was cooking, I felt as though I had some space. I could go and chat to DS without having to be rude to DP. It is not only his children that are demanding of my limited time, it is him too!
I do wonder if he would have been better off with a woman with a lot of kids of her own. Someone who really loves kids. Although having 7 or 8 kids would be a nightmare in a lot of ways, (transport, cooking, bedrooms) at least both parents would feel "equal". And that is certainly not what I have been feeling.
So, the next step is to discuss making time for me and DS as a family. I think I will swap some weekends around with my ex, I am going to take DS out on my own and I am going to expect DP to take his kids out on their own. I expect the kids will kick against this a bit to be honest, and I have to focus on the right reasons for doing this. Ultimately if something doesn't change, we are going to split up anyway, so we have to try to make it work.