Dave .......if you check out the website for HMS FLYING FOX ( put in a search on google or msn and look in history section) there is a photo on there of HMS FLYING FOX that the Royal Naval Reserve used until 1972 which say's she was an ex sweeper. Totally different looking than your photo.
I belive that the boat in your photo was connected to HMS FLYING FOX in that she was possibly used as the headquaters for the reserve ( before they went shoreside) and the sweeper was used for sea going training. As I live fairly close to Bristol and have travelled those roads many times I am pretty certain the two of them were at Mardyke berth just off the Hotwells Road under Clifton. Right opposite the dry dock for the SS GREAT BRITAIN. The dates would certainly tie in for this and Cardiff being only across the Severn......I will try and find out a bit more when I bump into the right people.
Arnie' here.RN Sloop HMS FLYING FOX. She became a training ship for the Bristol Division of the Royal Naval Volunteer Reserve in 1924 berthed at Bristol. Bristol Division became the Severn Division after WW2 and HMS FLYING FOX became a shore establishment in 1972. Dare say this was her fate.
She look very similar to HMS PRESIDENT moored in London which was also a sloop, HMS SAXIFRAGE I believe.