Unseen Magic by M.J. Putney

Jane Macrae is a Guardian, a member of the magically gifted families who quietly use their talents to aid humankind. During the darkest days of WWII, Jane’s abilities make her invaluable to British military intelligence. But when a powerful instinct sends her home to Scotland, she doesn’t expect to be sought out by a wild-eyed Canadian RAF pilot in desperate need of her help.
For Wing Commander David Sinclair, fighting the Battle of Britain is challenge enough. Becoming Warden of the Grail when he visits his ancestral home in Scotland is way, way outside his experience. Then his newly discovered powers lead him to a cool-eyed Scottish beauty who must become his partner in retrieving a stolen mystical treasure that could bring victory to the Nazis and destroy Britain forever.
Only if they survive might there be time enough for love….