In Biggles at World's End, HMS Petrel was a frigate despatched from the Falkland Islands to the Magellan Straits to recover the gold which after Biggles had discovered its location. When the Petrel called at Punta Arenas, Algy who had stayed behind met her Captain Anderson and briefed him about the situation--Biggles having gone on ahead to confirm the location of the gold. Captain Anderson, noting that a Russian whaler was in the vicinity, decided to press on to rendezvous with Biggles, arriving just in time. The Russian whaler had actually arrived a little earlier at the scene and might have beaten the Petrel to the gold had it not run aground. As it was, the Petrel was able to send a shore party led by Lieutenant Mason to recover the gold unchallenged.
The Petrel is a fictional ship. There was no such frigate by this name in the Royal Navy at any time. No details are given about its appearance nor was its class given. However a close fit might be the Black Swan class sloops which were built during World War Two. These were subsequently reclassed as frigates in 1948. Many of the ships in this class were named after birds, e.g. HMS Starling.