He's on over £70,000 a year ? What is he a banker ?
What he has been paying you does seem a drop in the ocean if that's correct and he is taking home £1,250 a week.
The CSA do have an upper earnings limit though where if the NRP earns more you have to get a court order for the amount over the CSA top limit. I think that applies to those earning over 100k but you might want to check it out.
Is there a figure in the middle of £250 a week and £300 a month that you would be happy with ? Could you come to some agreement by negotiation ?
I only say that because not only are the CSA about as useful as an ashtray on a motorbike they can and often do cause huge rifts and conflict that could affect your children and you for a long time to come. The words useless, incompetent morons are often used about them by parents on both sides of the fence.
What if your ex partner gives up his job as some men do to avoid the CSA? Or he pays intermittently, just enough to avoid the CSA taking action but not regularly enough for you to live comfortably ?
I have friends in both situations - one has a lovely letter inforing her that her ex owes over £70,000 in arrears but as he made 3 payments of £175 in the past six months the won't take action just yet.
Another friend of mine had an ex who moved to Ireland to avoid paying for his son.
My husband had the opposite - he paid religiously and fairly, his ex went to the CSA and ended up with less because my DH had been overly generous. She is now spitting feathers about it - not that you should find that happens in your situation though.
I would say if you possibly can do sit down with him and possibly a mediator too. Show him what he would pay through the CSA, tell him what he pays now is not enough and try and find an amicable path if you can.