This is for all you kind people who have provided support to me on here, I thought I should update you on what is happening for me and my situation.
Around 2 months ago, I announced to DP that DS and I were leaving him and that I was moving back to my house, that I wanted and deserved some control over my own life and that of DS life too. There obviously followed lots of tears, anger and upset. My house was still tenanted at that time but only a few weeks away from being empty. I started making all the necessary practical plans.
We carried on going to Relate, and we moved our appointments to weekly rather than fortnightly as I thought it could help us to move forward and might persuade DP that I was serious about continuing a relationship together where I could stop being upset about his behaviour and we could go back to enjoying each others company without the fraughtness of living with the constant trauma caused by his parenting/difficult ex/spoilt children.
My house is still empty but I am not living there. DP has had a BIG, BIG shock. He has had to open his eyes and listen and has heard not just me, but the counsellor telling him that no other woman will put up with his parenting style and that he is teaching his children some very bad lessons by not parenting them consistently and fairly and he is failing them and himself.
He has also been on a parenting course run through the courts and I can honestly say he is like a different man. He has admitted he has had to learnt to parent his DSD who came to live with us and not be "weekend dad" anymore, he has asked for my support on how to be a "proper parent like you are". We have regular, consistent contact set up between DSD and her Mum despite her whinges of not wanting to go because it's boring or other such insignificant moans. DP has pushed her to do it despite feeling protective and scared of losing her because he knows that is what is best for her. He is also pulling her up on her manners and lack of respect and doing this with the other DSC too, without any prompting from me.
I, in turn am also standing up to the children and not putting up with rudeness (and then running to DP to complain). I have learnt to put myself first and that I should expect respect and manners from ALL the children and make it happen. Eldest DSD, who I found to be so incredibly rude in the past, is like my new best friend now! 
It's not easy and it's not my "default setting" but I am trying and am happy with the results I am seeing.
I have had many chats with DS and he is happy to be living with DP and with DSD. A lot of my feelings of "losing him" are because he is growing up and changing and also because of my own issues and insecurities. DP is working with me on this and ensuring I get some SC free time with DS every week and accepting that is what I need and that DS needs this too.
We are both very aware that DSC have issues with expressing emotions partly because of the constant warring between their parents (they have told us they don't give any opinions for fear of causing another argument) but also because DP and his ex never show their feelings to the children. Both of us are trying to address this by talking and involving the children in things and talking about our own feelings and being more open.
DP is standing up to his ex and doing his best to take some of her "control" over our lives away. She is famous for not replying to e-mails or ignoring comments in the contact book and keeping us waiting not knowing what we are doing. DP is now giving her a chance to respond, and if she does not, tells her that as he has had no reply, X will be happening as per his original suggestion. All this has been off his own back. He has also said that going to court after 7 years is the best thing he has done (can't say too much as it's ongoing at the moment) and it makes him feel that he has a say and rights and is not beholden to his ex's whims anymore.
DP's relationship with DS is much, much better and I think this actually has more to do with DSD living with us than anything else, but I have stopped feeling protective over DS because of DP snapping at him because DP now makes real efforts to communicate with him and I feel that he is now a positive influence on DS. I also now feel comfortable with DP correcting DS on his behaviour because he is also being positive with him. DS has commented to me (with no prompting) that he has noticed that his relationship with DP is much better and I have witnessed the results in that DS now really talks to DP rather than just grunting at him!
So, to sum it all up, I feel positive about a future living with DP. We can really talk about things now and DP has completely lost the "brick wall" that I used to hit everytime I tried to communicate my feelings. However, I do feel that our relationship is like a little, delicate plant and it's going to need a lot of nurting and care to grow into a big old tree.
My house is going to stay empty until after Christmas, when I will then (all being well) look at getting tenants for another 6 month tenancy.