Danny, the oldest reference that I have to hand is the AA Illustrated Road Book of England and Wales, Fourth edition pub 1966. It shows the coast road twixt Hayling and Emsworth as the A27, as it remains to this day, similarly as does the London to Brighton A23. Reluctant as I am to suggest such a thing, has the memory perhaps very untypically played up this time?
Pom Pax, you are absolutely right, Sir. The same tome clearly identifies the "settlement" on the east side of Thorney Island as West Thorney. I can only claim in mitigation that it was never referred to as "East" or "West" Thorney within RAF Thorney Island, but merely as the Officers Mess, Sailing Club, Church, or whatever. In calling it "East Thorney" I was of course referring to its geographical location rather than its given name. There, that's me off the hook now, isn't it?... Isn't it?