If, after the previous steps, you still have a bill outstanding, there are a couple of options that may be suitable.
First of all, do please check whether you are entitled to Legal Aid. The intervention of a solicitor may help persuade HMRC the case is too much work to continue with.
The other possible option is pleading hardship. If your financial circumstances are very tight, and you can prove this, tell HMRC it would cause you hardship to repay the overpayment and they will perform a means assessment. This basically involves taking details from you of your expenses and income (usually done by phone but they may ask you to send proof of your expense figures). They have allowable amounts for each bill and will then deduct the allowable costs from your income. The remaining figure is considered your ‘disposable income’ and HMRC will allot up to half that figure as your monthly repayment plan. If the amount is very small or you have no disposable income HMRC will agree to delay recovery until your ‘circumstances have improved’ (note; it’s pretty unofficial as to whether HMRC will ever actually come back to you should they agree you are currently in hardship). You don’t have to be in dire straits to request a hardship assessment, but it will only get a bill written off if your finances are very tight.
If you still have a bill, can’t get a write off agreed and can’t afford the repayment plan HMRC have decided, do demand a hardship assessment to get the monthly repayment amounts reduced. HMRC can’t demand a higher rate of repayment than their hardship assessment allows for.