Sundials usually show time by a shadow created by the gnomon.
The gnomon is the part of the sundial which casts the shadow acting as the clock hand. Sundials are usually delineated to read Solar Time.
Solar time changes slightly from day to day due to the tilt of the earth and its elliptical orbit around the sun, where as 'clock time' which we use on a daily basis in our lives is based on each day having 24 hours of equal length.
The difference between Solar Time and Clock Time is called 'The Equation of Time'. Sundials show Solar Time so depending on the date you need to add or subtract a few minutes to get 'Clock Time'.
During British Summer Time, you need to add one hour to the time shown on the sundial.
British Sundial Society
The British Sundial Society was establised in 1989 and seeks to promote the knowledge of the science, history, art and craft of sundials. As well as maintaining a database of sundials in Britain, the society actively publishes research in their quarterly journal The Bulletin.
More information at membership details can be found online at http://www.sundialsoc.org.uk