You are correct, she was a Cable Layer. By the way her name has an "S" on the end. She was one of four cable vessels built by Swan Hunter. She was launched in October 1943. The other ships in the class were Bullfinch (1940), Bullfrog (1944) and Bullhead (1944). I have no idea why she has the odd name out.
They were 1,524 grt, 252 feet oa, twin screw with steam reciprocating engines giving a speed of 12 knots. During the war they carried 1 x 4 inch and 2 x 20 mm guns.
Bullfrog and Bullhead were transferred to Cable & Wireless in 1947. The other two remained in RN service until the 1970s.
I cannot find a pic of St Margarets, but I have tracked down a photo of Bullhead.
St Margarets was sold to Pounds of Portsmouth and lay alongside the motorway into the city for several years. In the early 90s St Margarets left Portsmouth under tow with the former tank landing craft/admiralty trials ship Whimbrel bound for Italy and rumours of a conversion to a sailing vessel? Not much is known after that, but apparently the Whimbrel foundered at some point during the tow.
in the early 60's the bullfinch and the st margrets had MN crews and I believe they were involved in laying cables to track submarines . They used to spend a lot of time creeping around the west coast of Scotland ,perhaps they were just on a pub crawl .