Research on Covenant Sites
Thadeus remembers a story from his early years, about wizards settling on a rocky island in the East Anglian fens. Everybody knows where it is, but they fear the snakes that live there. There is no covenant there today, though, and the library does not have the sources to confirm the story.
From his magic lore, two things spring to mind,.
1, Norse wizards are rumoured to harvest vis from rocks shed from the Wamses every year.
2. The druids is the dominant tradition of British wizards and witches, and they have several sacred places, often copses or groves, where they gather at celestial mark days. Using clues from the libraries, Thadeus thinks he can find one or two in the Pennines (which btw includes the Peak District and stretches North about to Hadrian's Wall).