See if you can pick holes in this, politely. I have no problem with alternative views but frankly when my parenting is called into question I do get pissed off and snappy.
So Plan B.
Buy a house and enable exDH to have 50/50.
His current reasoning for not having the kids more is that he had nowhere to take them. Currently does every other weekend in a hotel room. I'd charge him the mortgage as rent. He'd be doing well out of that arrangement. At least a £300 a month saving.
Disclaimer : I'm a darn good mother compared to what I received in the way of parenting and if it's not up to your standards, keep it up yourself until your child has married and avoid drug use/stealing cars/dropping out uni etc because it can and does happen to many many people.