To put the £400 you pay per month into perspective as we’re mid way through the 6 weeks holidays. £400 is the cost of putting twins through holiday club for one week where I live - I know it can be a lot more in other areas. £400 won’t cover everything.
If you really want your kids more, go to through the correct channels and get a 50:50 agreement. There will be no monthly payment then as you’ll both be sharing the care of your twins equally. You can sort out any wraparound care as you need on your days/weeks.
You will need to come to an agreement in terms of costs of clothing, uniform, footwear, school supplies, extra curricular activities, school lunches, school trips etc etc. Either one of you pays, keeps a tally and the other one pays over half or you split who covers what.
Another big one to factor in is school holiday cover. Kids are off for nearly 14 weeks. With 50:50 you’ll be responsible for 7 of those weeks to cover from your annual leave or unpaid parental leave. The time you can’t cover, you’ll need to book holiday clubs and pay for them. If you’re lucky, family may help. It most likely won’t match up with school hours therefore they’ll be some juggling
I’m sure someone else may be along shortly who can let you know stuff I’ve missed out on in terms of costs when your transitioning to 50:50