When is an island not an island? Last year I travelled to all four corners of England, Wales and Scotland to find out, becoming probably the first person to visit all 43 tidal islands that can be walked to from the UK mainland. Perhaps the most frustrating was St. Catherine's Island, with its Napoleonic fort.

I'd visited it once before, in 1968 when we had a family holiday in Tenby. Aged seven, all I recall is looking at the island as we played on the beach and visiting a zoo on it. Since this closed in 1979, various plans have been proposed, but none have come to fruition and the fort has stood empty, gradually decaying and closed to the public.

My email to the island's private owner elicited a clear no to my request for permission to visit. Not put off, I caught the train to Tenby to investigate. Unlike many tidal islands, St. Catherine's doesn't have a causeway, however formed from a huge mass of limestone, it stands on the beach, aloof from the town and when the tide is high is most definitely an island.

A multitude of signs warned of unsafe structure, falling rocks, cross currents and danger of being cut off by the tide. Private signs abounded and visitors were clearly not welcome! The only way onto the island involved climbing over rocks and two gates. I was prepared to risk arrest, but not a broken leg. My visit to St. Catherine's Island therefore extended to just the 19 steps to the padlocked gate.

St. Catherine's was once a place for religious meditation and solitude, with a small chapel on the summit. Its Palmerston fort never fired a shot in anger, becoming a luxurious summer residence for the wealthy Windsor-Richard family. The main hall was carpeted with animal skins from Asia and Africa, and decorated with glass cabinets, hunting trophies and stag heads. What a shame that this historic island lies abandoned on the Tenby sands.

With five chapters on Welsh islands, including one on St. Catherine's, No Boat Required takes the reader on a unique journey around Britain to explore islands, some familiar, but most which few of us know exist and even fewer have visited. It concludes that our tidal islands are special places, many with fascinating and amusing stories, and each one of them different.

• Priced £12.99, No Boat Required is available from Tenby Bookshop.

Peter Caton