This ford across the river is thought to have existed in some form or another since prehistoric times. Stepping stones were first recorded in 1841 and the ones you walk across now were dedicated by the UK Prime Minister after the previous set were destroyed during the second world war to obstruct potential invaders.
The stepping stones are great fun. In summer, children play in the river. After heavy rain, rising water makes the crossing impassable. However, there is a bridge further down.
Interesting fact, the stones were removed during the WWII to prevent an invading force from getting to London. The river is about 800mm deep.