I'm a retired accountant and can't agree that anything about tax is simple to many non-accountants or well-informed numerate folks. Some people find handling numbers doesn't come naturally to them, just as I can't paint, draw, write poetry or sew anything more complicated than a button!
The easiest thing to do is add up all the sources of income in a tax year (ending 5th April annually), using the Gross (before tax is taken off) figures, ignoring interest from ISAs (or other non-taxable income such as interest from some National Savings investments) and the first £1,000 interest from other savings, and deduct £12,570 for the annual tax-free personal allowance. The remaining figure will be taxed at 20%. If this figure is more than the tax already paid on the various sources of income then the balance is payable. Yes, apply to pay in instalments if it is difficult to pay all at once.
Your mum should have had communication from all her sources of income for the relevant tax year which will show the gross amount earned and the tax deducted "at source" - and as others have said no tax is deducted nowadays from interest received. A mistake I believe especially now that interest is not as insignificant as it used to be.
Good luck with it all.
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