When I bought the photograph I was told it was supposed to be a Turner - but, the early Turners are all fairly well documented and it's not one of the first eight road cars. By the time the 803 came out there was a standard factory body. It might be a Turner, but I did some digging in this area and couldn't find anything that matched.
It's certainly not a Rochdale GRP body, although it's possible it was one of their early aluminium attempts.
The registration number, apparently, is a prewar issue.