It transpires that the error is with the National Grid database, where my electricity supply number is registered with an address that does not exist. (242 lower floor instead of plain old 242).

EDF and Scottish Power are now playing the "pass the buck" game. So I told EDF on a call that was being recorded that as far as I'm concerned, I'm their customer so it's for them to sort out, and I did not consent to the transfer of my account. Siiigh.