My advice would be if you can, to find the cash and pay it into your account asap, so that your bank does not attempt to make this payment, reject it and then charge you somewhere between £20 and £70 for the privilege. Alternatively they may pay the transaction and take you into an overdraft (or extend it) and charge you a lot of money for this.
Contact your bank and see when it is going to be processed (if it hasn't already been rejected) and do your best to fund it.
This may be by asking for an overdraft or extension to cover it (which as authorised wouldn't cost as much).
Good luck.
Sparky.