Question About My O2 Deposit That Was Paid Into My Billing Account Today?

Upon signing a mobile phone contract I was asked to pay a £300 deposit that is withheld for 3 months and given back on the receipt of 3 successful bill payments. I got the amount paid back today (the day of my 4th bill) but it goes right in to my o2 billing account which means I’m unable to spend it which I wanted to do. The way I was told in the store was as if I’d get the cash paid right back in to my bank account. Has anybody had this same problem with an O2 deposit and do you know if there is a way I can get them to credit my bank account with the money and take it out of my billing account? Thanks.