A Premium Bond winner! Congrats, that's becoming rare these days.
It would seem logical (to me, therefore danger) to treat NS&I as a bank/building society account, list it on Sch. B, and treat the win as you would interest from an ISA (it is tax free, isn't it?) for 1116. Wouldn't the test be how is the account (and the location of income on 1040) declared on 8938?
As long as you declare it, and list it as appears correct to you......you're filing with all best intentions. If it's wrong, you file a 1040X, but there surely wouldn't be any penalty threatening problems. It wouldn't seem to be the type of account that would require an additional special form.