Lying east of Penzance, in the bight of sea between Land’s End and the Lizard, a rocky tidal island rises from the water. Now crowned with a castle rather than an abbey, St Michael’s Mount is the Cornish cousin of its cousin in Brittany, Mont St Michel. The story of how St Michael’s Mount was given and then lost by Mont St-Michel is fascinatingly twisty.

First, there is the question of whether the monastery on the island predated the Conquest. The monks there claimed it did, citing an ancient charter in which Edward the Confessor granted St Michael’s Mount to the Benedictines many years before William arrived in England. The problem with this claim is that the charter is, historians now agree, almost certainly forged. But, if so, it was forged by Norman monks who came over from Mont St-Michel after the Conquest. So why would Norman monks need to prove to Norman lords that they had long had title to a monastery that they might have expected those same Norman lords to give them?

Two answers have been proposed. Firstly, that by proving their ancient title to the land, the community on St Michael’s Mount would free themselves from the play of great lord politics, with its shifting alliances and occasional spectacular falls. With title to their monastery, the monks of St Michael’s Mount would be able to stand back and watch as spectators the clash of ambitions of powerful lords. The other, related, proposal is that the charter was forged as ammunition during a dispute with the Norman lord, Robert de Mortain.

De Mortain was half brother to William (they shared a mother) and one of his key allies. He was a member of the councils that agreed to William’s plan for invasion, he provided 120 ships and he fought at the Battle of Hastings. In return, De Mortain was given Cornwall. There is a charter, with copies surviving in Exeter and Avranches, which gives St Michael’s Mount to the abbey of Mont St-Michel in De Mortain’s name. A later dispute apparently developed between the monks of St Michael’s Mount and De Mortain over the ownership of the manor of Truthwall, and this may have led the monks to assert their ancient and immemorial rights – even if this required a little finessing of the past.

Whatever the truth of the matter, the Hundred Years’ War broke the connection between the houses on either side of the Channel. Henry V made the definitive break in 1414, giving St Michael’s Mount into the keeping of Syon Abbey. The monastery itself was broken by England’s greatest vandal, Henry VIII, when he appropriated the country’s monastic inheritance and the mount became a coastal fortress. It served as such through the centuries – pillboxes mark its most recent defences during World War II – but it is now one of Cornwall’s main tourist destinations, accessible via causeway at low tide, or by boat the rest of the time.