The Statute of Limitations probably applies & is that the debt cannot be enforced in Court if 6 years have passed since any payment was made or the debt was acknowledged. however, this does not mean that the debt ceases to exist at that stage - only that any Court action will be thrown out if the judge is satisfied it is statute barred.
It rather seems as if your mistake was to pay the �10K without getting a written confirmation that it was in full and final settlement of all liabilities. However, that's water under the bridge & doesn't help you now!
I don't know whether there is anything else you can do by way of getting an injunction or starting action under harassment legislation. To do either I think you would need a solicitor - and yet more costs.