Thanks guys for your advice. ASA are still investigating them (Case Ref: A13-242088). I am unfortunately very busy at the moment and therefore haven't had time to inform Trading Standards.
I did however start looking into small claims procedures (I've always heard good things about the way it's set up), and sent a quick, polite email as a precursor to court proceedings (which normally goes down well in court):
Order O-xxxxx was cancelled by Proviation one month ago. Refund still not received. Please advise as I must start legal action this week (week commencing 9.9.13) if refund is not received.
They processed my refund the following day. They included a screenshot of their refund authorization code, and said:
Your refund was processed automatically through the shop terminal around the 8th August. Sometimes they don't go through properly as we have changed credit card merchants and so they sometimes have to be manually entered. I have done this now for you and here is a copy of the transaction details on. [sic]
This is something that we are working on with our card merchant and we apologise that you have been affected by this.
Just thought I should let you know, as I had been losing hope (not being able to get through on the phone, reading this thread & seeing all of the others waiting for refunds etc). It is either a huge coincidence, or Proviation aren't fond of legal action.