How have you calculated what he 'legally owes you'?
What you should be doing is looking at the CSA calculator to see what child maintenance he should be paying. Obviously if he's intending to have the older dd overnight then there will be a reduction in the calculation for that.
That money is for your children and designed to keep them in the lifestyle they could have expected had you stayed together. He has to budget accordingly on what he has left. Just like you do.
As to the mortgage, I assume you will be buying him out at some stage..? Not sure of the legalities here as never had to split a mortgage, but this should be separate to the child mainenance really. Try and get it sorted as soon as possible. If he's no longer living there why does he have to keep paying? Unless you are making it part of the divorce settlement. You really need proper legal advice on that issue.
You then also need to see what you can claim in terms of benefits:
Have you applied for a reduction in your council tax for the single adult rate?
Have you applied for WTC & CTC? If you already do have you informed them you are now separated?
Have you checked whether you are entitled to any houseing benefit while on maternity leave and again after you return to work?
Are you planning to use OFSTED registered childcare? If so you can probably apply for the child care element of CTC to help with up to 80% of those costs.
Do you work full time or part time? If part time and on a low income you may be entitled to help with prescription costs, eye tests and glasses.
You may find the extra things you can now get mean you can afford to live on your salary, what he pays in child maintenance and benefits you receive.
However if it's not enough then you simply have to reduce your costs. And if that means moving to reduce your housing costs then you really must look at it with your head not your heart.
The other option is cutting back on swimming, food bills etc, or working more hours. I'm afraid that's what it comes down too.
So if your priority is to stay in that house and you simply won't move, and your income doesn't support your current lifestyle, then you have to be prepared to make cuts elsewhere to achieve that. I know that doesn't seem fair when he's the one who moved out and you don't want any of this. But that's the reality now.
So you need to insist he pays what the CSA guidelines says he should. If he won't simply ask the CSA to collect it on your behalf. If he can't afford to live after he pays that then he too has to look at reducing his costs, either with a smaller rent, downsizing his car, cutting his other expenses, working more hours....
It's not easy, but the money is for your children and he is obliged to support them. So really think long and hard before reducing what he pays. And if he doesn't like it then simply say you don't want it to be an issue between you so will put it in the hands of the CSA and then you no longe rneed to discuss with him (well except for the mortgage that is, but your solicitor can deal with that for you)