I read the discussion closely since I belatedly noticed a paragraph from a statement by NS&I saying: "For our 2, 3 and 5-year ... Bonds that are purchased or renewed on or after 1 May 2019, all interest will be treated as being received in the year in which the Bond matures." I had already made annual returns for previous years when, it appears, I should have waited until end-of-term before declaring the interest.
HMRC has clearly taken a firm stand on its own interpretation of the law, but I guess they did so for the obvious reason stated in the original post by rl11, that is, they will probably collect more tax as a result. It is a deterrent to anyone contemplating a longer than 1-year term, especially now that interest rates are higher.
The matter of interpretation of tax law ought to be decided by the courts but, in the meantime, I shall probably follow their guidance because (last time I had a brush with HMRC) the law said that I must pay whatever sum the Tax Inspector demanded, even though I might dispute the amount, before proceeding to appeal.