This person should complete self-assessment. HMRC are unlikely to have any clue about your total income situation unless you tell them.
do it before the filing deadline for the tax year.
‘but PAYE’ yeah, right.
If you have any other source of income, or are a higher or additional rate tax payer, or use gift aid, or … many other things, fill in self assessment. Then ignore HMRC when they say ‘you no longer need to complete self assessment’. Accepting that is a recipe for almost ensuring you’re paying the wrong amount of tax.
If you’re losing money to HMRC it’ll go unnoticed. If you’re underpaying, well…
edit as for paying tax on savings interest – if you’re sure you’ll be below the threshold no action needed. Savings accounts often have higher returns than ISAs and fewer complications. If you are likely to exceed the allowance then if you don’t need the money pay more into your pension. If you might need the money – ISAs. IANAIFA