I kind of think it is common sense that he should have to give up as much of his disposable income as the OP has to give up? They clearly at some point made a joint decision (or possibly not even so joint, from what the OP has said but in any case, a decision was made) that they would divide the work that had to be done to keep the show on the road between them in such a way that the ex husband would do the paid part and the OP would do the childcare part. Those parts are of equal value. I very much doubt that OP was sitting around all day painting her nails and reading her book with a ten month old in the house. In the extremely unlikely event that she was doing so, she certainly won't be once the new baby arrives. My heart goes out to the OP having to do this alone. I can only imagine that the posters who are minimising the OP's current situation must have forgotten that frightening and exhausting time with a newborn, and how it felt. The new baby is not going to sleep through the night any sooner, or cry any less in the witching hour for those first 12 weeks or so, just because the OP's husband has buggered off into the sunset. It's absolutely right that she should receive at least the equivalent of a living wage for doing his share of the childcare as well as her own. If she were to behave as he has, and swan off, then he would either have to do this himself or pay a 24 hour nanny. Including tax and NI, that would cost in the region of £15/hour where we live. 15 x 24 hours x 30 days per month = £10,800 per month. That should obviously be divided by 2, because they are equally responsible for the care of their children. So that is £5,400 per month that he should be paying to her on top of the "disbursement" costs of food and clothing.
There is something really offensive this idea that the poor husband shouldn't be disturbed in his right to start a "new life". "New life"?? Where is the OP's right to a new life then? How's she going to be managing that one whilst simultaneously caring for three children, two of whom are effectively babies? And OP, there is nothing at all wrong with expecting that your co-parent should be giving his all for the welfare of his children. Nothing at all. You would assume, wouldn't you, that anyone decent would do it without thinking about it, but the prevalence of alternative views on here does start to make you wonder. Should the OP's husband have a right to somewhere to live so that he is not sleeping in a doorway? Of course. Should the OP's husband have an overriding right to a beautiful apartment with a river view balcony? No, absolutely not. He has a right to a life. He does not have a right to a "new" life. He has a right to the life he built, with all the responsibilities and good bits and bad bits that entails, just like all the rest of us. If he has to live in a room in a shared flat in order to provide the best life for his kids that he can, then damn right that that is what should happen if it is necessary. I'd be interested to hear the views of the posters who are defending the husband on this. Er, are they honestly saying that they wouldn't be willing to do that for their kids, then...? Surely that isn't even touching the sides of what the average parent would be willing to do for their kids?
Sorry - that's a bit long and ranty, so apologies, but I'm really, really angry on behalf of the OP. OP, I hope that you're getting through this as best you can, under very difficult circumstances. Please don't take to heart all the awful stuff on here.