Sorry canpaywontpay let me get this straight.
You 'incurred a debt' because your partner did FT childcare? Are you mad? How much do you suppose FT child care would have cost you both if she worked? And would you have wanted your DS to be in FT childcare? The money you earned was your family's money, NOT YOURS.
You are earning considerably more, and yet you have not increased your payments? Have you been paying what the CSA would have assessed you as owing? If not, why shouldn't she involve the CSA?
You say she wouldn't sit down with you to discuss it. What reason would she give for this? It seems an odd response unless she fears being bullied or intimidated in some way, as her DS would otherwise benefit.
Clearly the arrangement you had was not 'perfectly acceptable' to her. You don't get to dictate this.
And this makes me the most angry. THE MONEY IS FOR YOUR CHILD. Not her. Not another mans son. That other child is irrelevant here.
And how are you any better? With YOUR revengeful choice. And YOUR tit for tat behaviour. You sound about 6. The only person who loses is your son, whose mother will consequently not be able to provide for him anything like the life he could have with that extra £450.
I suspect you have been massively underpaying, and are now using awful self-justifying arguments to let yourself off the hook of having to pay what you should have been doing all along, and it is your DS who will suffer.
He won't thank you at 18. He'll see you for what you are. You won't LOOK like a 'dead beat dad', if you do this you ARE a dead beat dad.