Thermo's right...probably easiest way....although Slogger option is strangely tempting.
If it were me, I'd demand the money back put in writing. Send it recorded delivery or at least registered post. If they don't do this immediately, threaten them with legal action, with writing to your local newspaper, Watchdog, The Sunday Times (great link Pip!) who are looking into these sorts of false charges, your MP and anyone else who might fight on your behalf or put the fear of God up them! Just kick up such a stink they'll want to refund you the money or know they'll be in for some stick. If they don't give you your money back, write those letters and follow them up with a phone-call if you don't hear back, then get the solicitor involved.
By the sounds of it, you've been given the runaround. If that's the case, your highlighting this will hopefully hit them where it hurts, with loss of business, inform others of these sorts of dodgy practices and hopefully your money back.