Hi mate, used to see the Petrel a lot in Devonport in the early 1980's, believed she was used for the university training boat then. Always berthed in 4 basin, close to where the RMAS berthed. Seemed to remember having to pilot her from the Hamoaze up to Ernisettle, because her gyro compass had failed !! remember she was painted standard light grey and her decks may have been dark grey as that was the time the RN was going from green to grey decks. Also according to my 1983 Janes, Kingfisher was the lead build vessel, the others differed in no crosstrees for the radar and no scuttles. As Nick says very similar to the Seal but bigger and the rear accommodation block was different and she mounted a 40mm on the rear.