The Priory of Finchelsea XIV

The English Chapel was deserted when Lil and Kathy reached it. The corpse was gone, and the way to the Tabernacle was open. Lil sprinted across the floor, jumped over the altar-rail and tried the doors of the tabernacle on the altar.
It was unlocked. She opened the doors and looked inside. In the dim light of the chapel she could see little.
"I've got a flashlight," Kathy offered. Lil shuddered.
"Don't remind me! What's the man got against me anyhow? Apart from my being American, of course."
"Your book The Sexualisation of Imagery. Flash is a devout Anglican, and he thinks that all the Gnostic stuff is blasphemous."
Lil groaned again.
"He hasn't read the book! I was arguing it was all absurd!"
Kathy shone her flashlight into the dark interior of the Tabernacle. Lil, who was a Presbyterian, felt a certain saisfaction in violating this piece of Popish furnishings.
There was a key there. It was an interesting sort of key, made of rock crystal with a Welsh dragon for its head. Lil picked it up and looked at it with interest.
"A key. The trouble is, I've got no idea what it opens. Try that blacklight torch!"
Kathy did. Nothing showed. Lil sighed.
"So we're stuck. Unless... shine it on the floor again!"
"But..."
"Do it!"
Kathy did. Again the now familiar words showed in the beam from the blacklight torch.
There was something she was missing, Lil thought. She, Lil Barton, brilliant college professor...
Of course!
"The second line!"
"Summer is acumen in?"
"No. we expected it to. Look at the second 'm'!"
It didn't read 'm' at all, it was an 'n'. The message actually said "Sumner is acumen in".

"Sumner!" Kathy exclaimed. "That must be sir Teabag Sumner!"
"Teabag?" Lil asked, amazed that anyone would have such a name.
"Yes. I don't know why. But he's a world expert on the Holy Grail! And he lives in Gower Street."
"Then let's get there - quick!"
Lil dashed out of the church, and Kathy followed.