Hello Clive, I'm Keith and happy to help you with your question. Here is the correct answer by the book. Your Mother is clearly working cash in hand on the black economy deliberately evading Income Tax [ha, ha, joke]!
The book says that she should register as self employed with her tax office and make annual self assessment tax returns. Tax Aid says: 'People who have income that has not been taxed at source, or not taxed at the correct rate, and on which tax is due are required to inform HM Revenue & Customs about the income within 6 months of the end of the tax year in which the income is received (that is by 5 October following the end of the tax year). HMRC will then send a tax return for you to complete. Such income would include, for example, rental income, self-employed income, savings income for higher rate taxpayers, and occasional untaxed income like eBay sales.'
Even HMRC do not really want to be bothered with processing tax returns which will generate no tax income and, as you say, your Mother will be below the tax threshold. Just to cover herself though it would be advisable to approach an HMRC Enquiry Office or your Mother's own tax office to ask them what they would prefer. HMRC these days are rather keen on imposing penalties for omissions the taxpayer didn't even know they had made or were not aware of the requirement, despite a clear advice that if the payments are not taxable no return is needed. I know it's a bit belt and braces, but I'm an accountant and tend to cover my back and be naturally cautious. In my opinion a letter should suffice. Don't try to phone, we all know the delay that will involve and a second class stamp is far cheaper!
I do hope I have helped you with your question. When all the chips are down I think you will find that your Mother will not need to do anything once the air is cleared. Your Mother is, I assume, claiming pension tax credits assuming that she is entitled. Her gardening activities may well affect these. Life is so complex isn't it? Always bear in mind Benjamin Franklin's dictum that in life there are but two certainties, death and taxes!