Answer May 2021
Click on the images above to see a full-sized image.
The clue is a sundial on the back of No 12 Hallstile Bank, hidden by the Council extension to Prospect House until the recent demolition. The second photo, taken from the Moot Hall and looking NE across Hallgate, shows the sundial in-situ, on the white wall abutting the red brick of Prospect House; the third photo is a close-up of that area. The fourth photo (on the right-hand side) also shows the sundial on the wall.
Thanks to the HLHS member (whose name I’ve forgotten to record) for the suggestion and a couple of the photos.