More tax credit renewal -childcare payment check

after reading the posts on here was not surprised when a child care check letter dropped through the door, my son goes to the after school club 5 days a week every week through out the term so its always the same figure etc what my problem is during the summer they have 2 organisations who run the camps as they split the holiday
My problem is its a pay as you go arrangement and I literally send my son with a cheque for the days he attends on a weekly basis and I've never had any Invoices as its a parent run scheme I've only got invoices for the term time. I will be asking for a letter from the holiday club to confirm the payment method etc but I'm very doubtful that they will have records of exactly how many days his attended. There were weeks when I've paid the full £150 for the week But I never called tax credits to amend as worried claim would be cancelled so just paid the extra myself am wondering now am I going to get shot in the foot exactly how through are they being as I'm guessi g this exercise is a to clawback as much cash as possible though I bet if I've been underpaid I won't get any back !!! have tried to call the hotline its a joke it just rings and cuts off.