Ex refuses to tell CSA we have paid her

my boyfriend has a one year old son with his ex partner, he began working in april and had been giving his ex £50 week until recently, although he never got receipts off her. she has now gone to the csa and he sent off 5 payslips on particulary good wage weeks so im sure the average they have worked out is wrong. they have also said that as he started work in april he needs to pay £550 back pay even though he has tried explaining she has had money.

we are now panicking how they are going to take this money off him as £55 is already going to be coming off his wage and much more than that we are really going to be struggling. do you have any ideas how they go about getting these lump sums???

thanks in advance for your help

Vicki phelps