I'm going to start by saying that I'm sorry because I haven't read the whole thread.
However, stardustnight, I don't think you are being unreasonable at all. The CSA calculation that a child will 'cost' 15% of a parent's income, and that subsequent children will 'cost' less than that again is laughable. Even just starting with the basics, a child needs a room to sleep in, and a bed, and light and heat in that room. They also need food, clothes, school uniform, packed lunches, day trips etc. And that's before you even start on the stuff that really they should be doing, activities, clubs, days out to theme parks, art galleries and the things that while it might not be essential is still hardly Little Lord Fauntleroy territory like music lessons, tablet, internet provision etc. I certainly spend a fuck of a lot more than 15% of my income providing that for my son, and I'm sure that's the case for most people.
I think there is so much negative shit in the press about evil sponging single parents and how they are bleeding the poor men dry that we lose sight of the fact that there is an entire army of women out there, getting by on the bare minimum, looking after the kids, raising their families, holding things together while the shit hits the fan and the blokes swan off into the sunset and that that is the real scandal. 15% is a derisory amount when it comes to what it costs to raise a child.
So, you are completely right.
As for where you go from here, I'm sorry if I'm repeating what others have said because I can't read all 27 pages and it all got a bit bogged down, but a good lawyer is definitely a first step. Also, do a tax credit calculation here:
www.entitledto.co.uk
Go to your CAB - they have welfare benefits advisers there that can help you get whatever you should be claiming. They also could be of help in the future if you need something replacing urgently eg a washing machine - every area has charitable funds set up by benefactors where people can apply when in need - it might be if you live in a certain postcode etc you can avail yourself of charitable help. Find out about it and take it.
Contact your local authority about your son's music lessons - if he gets put onto free school meals after the split, some authorities run bursary schemes where they would pay for tuition within/after school.
But mostly, congratulate yourself that you are doing the right thing, but that it's also ok to feel pissed off and cheated that even though you've made this huge step things aren't sorted out immediately and to feel daunted by the battle ahead of you. But know that you can get through this, and your kids can get through this.