Not sure about a cathedral hull, but it could be a Shetland.
Definatly not a Dolphin, I used to build them back in the 70s and like tr7v8 says they were bs 1088 marine ply to start, until we moved to GRP hull's, but they sold out and were bought by a Southampton Company long before 1980. Ive still go pics of the Dolphin Range if anyone Interested.