I had a slightly different problem with a company last year, in the end I resorted to the 'Distance Selling Regulations'
Every letter sent was saved on my computer together with every e-mail. I sent an e-mail and letter at regular intervals basically telling them they had x amount of days remaining to issue a refund prior to my taking action through the small claims court. With about 3 days remaining of the 30 day period they contacted me to say they would issue a refund within the next 7 days, I simply replied and informed them they had 3 days left before I took court action. They issued the refund on the 30th day.
Just keep hammering away at them and be prepared to take the matter to court if need be, chances are they'll hope you're bluffing so if the worst comes to the worst surprise them.
edit, I'm pretty sure you must inform them in writing, make sure it's Recorded Delivery as Dave says.
Edited: 12/03/2011 at 20:13