Recently left o2, direct debit question

Gueld

I cancelled my o2 contract at the beginning of this month, choosing to have my number transfered to a payg sim and to recieve a refund for the rest of the month as the operator told me I was paid up to the 1st of August. However, my direct debit still came off a week later, I'm guessing because it can take up to two weeks for it to cancel?

Anyway, what I'm asking is, does anyone know how long will it take for O2 to refund me? Will I have to chase them up?