What hasnt been said so far is that a threat of a Solicitors letter is not enough to make you pay – they need to take you Court to enforce the debt before they can enforce payment.At the moment they are harrassing you to make you pay. They will keep doing it until you give in, or you make them stop by threatening them with Court action of your own.
It is very unlikely that they will take anyone to Court for such a small amount.
Firstly, deny that you owe the amount. If you say there was a dispute, ask them for their proof that you owe the debt. It is unlikely that they will have any paperwork supporting their claim.
If, in the very remotest of chances, that it goes to Court (it wont) then give your side of the story, and say the amount is still disputed with the original Claimant.