A question for HMRC: do you believe you have given banks and building societies a tool and guidance that actually allows them to report the interest only when it is accessible at the maturity of a multi-year bond?
The guidance for them to report is here: https://www.gov.uk/guidance/how-to-complete-a-bank-and-building-society-interest-return
"Gross interest amount
Enter the amount of interest paid or credited to the account for the reported tax year."
Note that "or credited". The problem is that the multi-year accounts may *credit* interest to an account each year, but it won't be accessible until maturity. You instruct them to report the interest to you, nonetheless, and there seems no way for them to mark it as "but this is not accessible in this account".
Do you think that all banks with accounts and interest only accessible at maturity are not telling you "the amount of interest paid or credited to the account for the reported tax year", but are instead, because of your guidance elsewhere, holding back the reporting of interest until maturity?