To round this out, and maybe with some useful information for others.
I had a long interaction with someone in BA chat who insisted it was a premium economy ticket and I should go back to Amex travel, from whom I bought the ticket. Somewhere in the flow he let slip the BA record locator. Once I logged in with this the sector showed as club world and I could select a window and I’m happy.
I guess I’d assumed that because the QR record locator worked on the BA MMB there wasn’t a BA one.