HMRC can go back 20 years if there has been deliberate or negligent failure to disclose income or gains.
Is this a hypothetical question, or have you got a skeleton in your closet? If so, the best way forward is to voluntarily disclose it - but get some professional advice first.
Presumably since you refer to making payments, you've been previously assessed by HMRC anyway for a previous capital gain. You'd probably have to be declared bankrupt to remove the debt if such a point has been reached,