Hi TLB
It looks like a prototype fire float based on a seaplane tender, there were several produced before the war and just after, varying in length from 38ft to 56ft. A lot of them looked like the traditional 46ft Fire Float 93 and 94 but had subtle differences. I'm guessing that there is only so many ways of fitting a fire tender to a old hull! Christian might have the details from the RAF Marine Craft Directories which list all RAF fire float designs, I'd do it for you but there is a few hundred miles of empty water between me and my copies!
