Firstly, do not acknowledge to the creditor that you owe the debt. If acknowledgement of the debt in writing is made then you will be liable to pay the debt back. If any payment is made towards the debt, even after the six year limitation period has elapsed, then the Limitation Act 1980 will no longer be enforceable and you will be required to pay back the full amount.
Secondly, write to the creditor quoting the Limitation Act 1980 but do not acknowledge the debt. This will inform them of your intentions and make them aware, if they were not already, that they may no longer pursue you for the debt.