Circa 1965/66 we had 
Then 1970s (HMS Amazon 1974) and throughout the 1980s these; I gather second larger circle was needed for larger helos such as Sea King. Book Ships of the RN 1981 has pictures of a County with 2 circles and frigates with one. 
Much later (date ?) the circle has shrunk and diagonal lines have been added, maybe for the Merlin 
Where is the Observers Book of Deck Markings when you need it!
Message Thread 
Evolution of RN/RFA helo deck markings? - Kelvin November 30, 2022, 16:06:20
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