British Summer Time in Rydal

This is the sundial outside Rydal church. It was taken when the Sun was highest in the sky so the sundial said 12 noon. However my watch said 1pm. British Summer Time. What I need to think about next time is whether an accurate watch set to Greenwich would be out by up to 15 minutes because we are west of Greenwich. This sundial didn't come with the graph like the one at Amble so I need to be more observant if I go again on a nice day.