If you are on the prime meridian at midday GMT then you would expect the sun to be due south. However this is not the case and the sun will be slightly ahead or behind of where you expect it to be. This difference in time between what your watch reads and the position of the sun is called the Equation of Time.
During a year the equation of time varies and the sundial can be ahead by as much as 16 minutes 33 seconds (around 3 November), or behind by as much as 14 minutes 6 seconds (around 12 February).
Only four times during the year will your watch and the sundial correspond, around 15 April, 13 June, 1 September and 25 December.
The reasons for the difference are the Earth’s elliptical orbit around the sun and the tilt of the Earth’s axis in relation to the plane of its orbit around the sun.
Homework
Place a straight stick or pole about 1m in length into the ground and leave it there for an entire year. Then at regular intervals throughout the year, at the same time every day place a smaller stick where the sun makes a shadow with the top of the pole. It must be at the same time of day that you do this so you will need to adjust for daylight saving time (if you do it at midday during the winter when we are on GMT then you'll need to do it at 11:00am when we switch to BST.)
After a year the small sticks will have made a figure 8 pattern - this is called the analemma.
The north-south component of the analemma is the declination, or the latitude at which the Sun is directly overhead. The east-west component is the equation of time, or the difference between solar time and local mean time. This can be interpreted as how "fast" or "slow" the Sun is compared to clock time.
Further Reading
First Ever Photograph of the Analemma by Dennis di Cicco during 1978-79
http://en.wikipedia.org/wiki/Equation_of_time
http://en.wikipedia.org/wiki/Analemma
http://en.wikipedia.org/wiki/Analemmatic_sundial
Congratulations on FTF - Kikikaty & OnlyMeUK