Instead your best argument would have been to try and argue that the debt is out of time to be pursued, however looking at the dates it may still be in time.
The Limitations Act 1980 states that a debt based on simple contract, like in this case, must be pursued within 6 years of the date the cause of action accrued. Generally, if no specific repayment date was agreed, it would be from the date a demand for repayment is made. If you stopped payments under the contract in late 2010, then that is the earliest when the cause of action would have started. In this case they would have had until late 2016 to make the claim, which they have done so they are likely to be in time still to pursue this. Therefore, it may indeed be best to consider repaying this debt, knowing that at least they have only asked for £50 a month rather than a lump sum payment.
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