Assuming you have taken the reasonable steps so far of talking to them, explaining your position and requesting a full refund, next step is to write to them explaining the same, give a reasonable amount of time in which to make a refund (say, 7 or 14 days), and that if yo udo not receive it then you will pursue the matter through small claims court.
Don't start quoting BS about cans of whoop ass, thats for sure.
If the item was over £100, and you paid with a credit card (not debit card), then you may well be able to pursue it from your credit card company, who in turn can then sort it out with Westec.