In reply to aln:
Just to agree with some fine points above, make sure you pay by bank transfer or DD, so you have a record.
Keep a diary of when you have the children and what you buy for them.
I was left in the position where she bought everything and I had no money to buy anything, even mundane stuff like shoes and school uniform. We lived in different town about 30 mins away so school day staying over was too much bother and not a stabilising influence. so that left every other weekend. This isn't enough to count against what we paid so even doing the right thing cost me extra. I could have claimed travel expenses if I'd used public transport, but not for the car. She got to keep all the child benefit, even when he came to live with us!!
There were so many shit rules, but this was 20 years ago so hopefully things might have improved ha ha!!
Best sort it yourselves but have something in writing, just a letter will do, I drafted one and each of us sent it to the CSA (as it was then). It may be amicable now but the futures a long way away and things can turn horrible quite quickly, you get another woman and that can cause all sorts of resentment, she gets another bloke who can change the dynamic, imagine any ex relationship and then magnify what might go wrong by 100.
So it's best to just cover yourself, good luck.