Childcare grant and income /overtime

We currently get childcare grant and get the full amount we are entitled to 85%.
We have kept you up to date with expected income. But after talking to another student we are worried.
Is there a sudden cut off for childcare grant or does it taper off?
We are worried if my husband does any overtime we may tip over into not being entitled to childcare grant. So therefore have to pay back what we have been paid for this year? (This is a lot of money)
Just to reiterate we have kept SFE updated with any changes into income. But just need to know when he does overtime here and there could this suddenly mean we are not entitled to CCG that we have already paid out to our childcare providers? Is there an allowed amount above an estimate before we are no longer entitled? Or that some it you earnt 1p. Above the threshold so therefore you weren't entitled to CCG this year and must pay it all back. Thanks