Bees "he was clearly mad that I was not full of appreciation for his gesture." He is a bit delusional isn't he?
"what he doesn't know is that I've already told my dm who is supportive of me but who advises me to keep my mouth shut til I can buy him out in may"
I am not sure why you or your mum are so set on keeping this awful man sweet and biding your time. What if he does some really bad emotional or even physical damage in the next two -three months?
Re "keeping him sweet even by deceiving him (which i understand but not really ok with" What are you not OK with? Deceiving him? I'd have no qualms about that but would worry he will see through it and get really angry.
"He rang his uncle so that I could hear him in the other room and told me. I then left the house and rang his uncle myself and explained and apologised (I am so embarrassed)."
I just do not understand what all this pussy footing around relatives is about. Why care what his uncle does or does not think. You and your kids are what is important.
"I want to buy him out mainly because this is my dc's home, we are settled here and I just wanted them to have as much stability as they can. It's a really sought after location and we have lots of friends here. (Although maybe I won't have after he's finished)."
It is 100% your choice and your call, who am I to tell you what to do. Honestly I do understand the desire for stability. But I wonder if you will feel safe and happy in the house you shared with a man who has made you so unhappy. Maybe long term you may move anyway, just a thought.
Even if you will but him out in May, why do you need to stay there, could you rent a small place for a couple of months. I know it is not easy as most leases are 6 months or longer but worth at least asking, isn't it. If you do cover your tracks.
If this horrible man is able to turn your friends against you, then they are not really friends. Whatever your reason for wanting to end your marriage, it is valid, this is your life. It is none of your friends business!
You do seem to be worried about everyone else but you.
Yay, I am so glad to hear you say "I am going to ring women's aid tomorrow."
So, yes do please speak to Women's Aid. I feel strange advising you, I have no experience in these matters aside from two friends who both fled unhappy, abusive marriages. The professionals know best.
I am afraid "I was utterly mortified", fear of embarrassment is keeping you trapped under his thumb. please do let the people at Women's Aid know how much he holds things over you.
"Ds was upset on the ride there, questioning me, blaming me and saying that I should give him a chance as he has 'changed' in the last month and not said anything nasty to him."
So, with respect, your son (who does not understand all the facts of abusive men) cannot coerce you into staying with this man, nor should he be put in a position to do so. I was delighted to read that once he knows the facts he doesn't want to do this. He is one smart 10-year-old. 
What you said to your son was spot on.
What your son said is also spot on.
So YES TO WOMEN'S AID, do not rely fully on your mum, us or any other person to advise you, we do not know the full story, only you do and you are so worried about what others will think you are allowing this to cripple you, IMHO. So please do speak to a professional at Women's Aid and tell them the full story. We are all rooting for you.
You have not derailed. [hugs] XXXXXXXXXXXX