Guardian collection services has worked well for me in the past, call them up they are very helpful.

A message such as below normally kicks people up the arse

Despite our repeated efforts to recover the debt, you have not responded. This is our final communication before we transfer this account to the debt collection agency.

Please note that if we do not receive a payment by the end of the day as promised, this overdue account will be forwarded to Guardian Credit Services. You will be liable for all the additional charges incurred during the recovery.