Do tax credits pay in advance or in arrears ?

I know you are supposed to inform HMRC when the hours change has happened - but in my case I'm going from 28 hours to full time on the 29th April.
I'm due a payment the previous Friday 26th April.
Does anyone know if this is for the previous month I've just worked at 28 hours or for the upcoming month ? as if it's for the month ahead ? it might be an overpayment so I'd have to pay it back ?
When should I ring the tax office ? I do not want to risk an overpayment but my full time hours wont start until end May (I mean when I get the first months' full salary) - so I cant manage for a month without the working tax ?