Identify a book or story where the protagonist(s) have a secret base on the island of Rockall?

I have a distinct memory of a fantasy or SciFi story where some of the characters in a story and some of the settings involved a secret base on the Atlantic island of Rockall.

This was probably published before the 1980s and might have been a spoof rather than a completely serious story (maybe, the memory is old).

Possibly The Master: An Adventure Story (1957) by T. H. White

It involves two children, Judy and Nicky, and their dog Jokey, who are stranded on Rockall, an extremely small, uninhabited, remote rocky islet in the North Atlantic Ocean. They find that it is hollow and inhabited by a mysterious person who aims to take over the world.

Not matching, it's the antagonist that has a base.

From the fantasy side, there are Anthony Swithin's The Perilous Quest for Lyonesse books.

However, there is also a vast, sweeping alternate-world version of the island in which it is a much more extensive territory, complete with its own history and customs. This was created by the eminent geologist William Sarjeant (1935-2002), under the pen-name Antony Swithin. It also brings in aspects of the lost continent of Atlantis, the drowned realm of Lyonesse, and the mythical land of Hy Brasil.

Four books of this epic fantasy were issued in paperback by Fontana from 1990-1993 under the series title The Perilous Quest for Lyonesse: Princes of Sandastre (1990); The Lords of the Stoney Mountain (1992); The Winds of the Wastelands (1992); and The Nine Gods of Safaddné (1993).



There isn't a ton of information about them, but one Goodreads review of the first book states

I loved this quartet from Antony. It tells of the journey of a young lad searching for his missing father just after the rebellion of Owain Glendower in the 1400s. His journey takes him to the mysterious archaepeligo of Rockall - a land of strange creatures and new adventures. Will he find his family ... read on.