When I said 'handwritten' it wasn't literal handwritten.
I meant it was a pre-printed form letter (carbon letter actually, I had the bottom copy) with boxes which were handwritten.
It did have the EDF logo at the top.
The account number on that matched the account number on one of the 'Occupier' bills I'd opened which in turn had the same meter serial number on it which Octopus were able to automatically identify from the address,
so I believe it was all genuine.