There's a huge novel that I think zi read here, but maybe bought later. And lost. Modern England. Ex SAS meets his okder mentor, who was a post wwii spy. Hero inherits a Swiss security agency, in a castle. Castle is attacked. Finds a series of caves with weii nazi treasures incl a v2rocket.
Attackers are a rogue element if the Vatican. Hero ends up marooned on an island for awhile. Enemy nukes England. Any clues?