Have you gone through the CSA to determine how much he legally has to pay? This would potentially go up if he wasn't looking after Martha overnight as much in the next few months.
Whilst I have big issues with the CSA, and certainly advise not paying through them and instead setting up a private monthly payment from your ex, the CSA will at least be able to tell you how much he legally has to give you.
There's a
CSA Calculator here which will give you an estimate if you know your ex's salary.
Of course, it might be that he's paying more than the CSA demand, or helping you towards the mortgage seperately from your child support, in which case I don't know what liability he would have to pay you.
Good luck with sorting this out. Money can cause so many issues with separated parents - I think it's important to try and not let this affect your relationship with your ex for the sake of the children.