Correct for longitude. For every degree east, subtract four minutes. To get mean time from meridian time add the Equation of Time. When summer time is in use, add one hour. This is all you need to know to put your watch right by a sundial, assuming of course that the calculation of its hour lines and its orientation are right.
Some sundials are wrong.
You can check whether they are at fault by starting with a watch known to be showing the correct time and working backwards. When you have grasped all this you will be as amused as I am to observe people looking at sundials and consulting their watches. I suspect the latter for they frequently make remarks which cast doubts on the accuracy of the sundials. For maximum entertainment stand close to a sundial situated as near as possible to Land's End, at the end of July when the difference between local solar time and BST is nearly one and a half hours. How latitude does interest the diallist I will explain next month.