Get aggresive !
The problem that you have is that the limit for the small claims court is £5000, so if you go down that route you will be £1200 out of pocket but there is little risk and to start the ball rolling it will only cost £62.
If you go for the full £6200 it will cost you a lot more and if you loose they can hit you for costs.
My advise is to pay a tramp to sit in the head office at Elstree untill they give you the money back, they will give up in hours if the guy smells bad enough!