Is a tax return compulsory for sole traders/self employed?

[mjb1]

My friend was made redundant a couple of years ago, and he's not worked since (lived off savings/redundancy payout). Now a company has offered him a couple of days (literally) of work on a consultancy basis (paid on an invoice basis, not PAYE). He's getting it done before 5th April so it should fall within his personal allowance for the current tax year.

At present he isn't registered as self employed and hasn't completed a tax return for a year or two either. He was hoping that he'd be able to just send a letter to HMRC declaring this one off income and that he's still well under the annual threshold for income tax and NI and avoid having to do a full tax return. So he phone their help line to ask, and they replied that he didn't need to do anything at all - apparently he doesn't need to register as self employed, and he doesn't need to inform HMRC as long as his income is below the personal allowance. That surprised both of us - I was expecting him to have to complete a tax return at the very least. Surely it can't be right that anyone trading below the personal allowance doesn't have to inform HMRC of anything at all?

He took notes of the telephone conversation, and is relying on that to cover him.