To my astonishment, I have just discovered that (subject to a handful of conditions) a person operating in the "informal economy" can re-enter the taxpayers' club by dialing 0845 608 6000 and coming clean.
If less than £2000 in tax is due, this will be waived if HMRC have no record of the person.
The limit may be over £2000. The HMRC man that I sopke to was not keen to give me a figure, but he confirmed that £2000 was okay.
For more detail, search the HMRC website for 0845 608 6000 (the Tax and Benefits Confidential helpline).
I was astonished because I understood that no amnesty had ever been offered.
Grumpy Old Wit