Manx Genealogy Archive 2

Re: Radcliffe of Knockaloe and Baron Fitzwalter.

footnote in Vic history
Lord Fitz Walter on succeeding found that many of the charters were in the hands of Thurstan Tyldesley, as executor of the John Radcliffe who died in 1513; but Thurstan professed his willingness to deliver them up, as soon as he was assured as to the heir; Duchy of Lanc. Plead. xiz, R. 1.

The Tyldesleys came over with Stanleys in 1405 - Thurstan Tyldesley crops up in some Manx records

You are correct in saying that hunting down the route by which the abbot story emerges would help - certainly it appears that Radcliffes were not in possession of Knockaloe until some time later.