made it !

The source is marked by this stone, the river only flows here in times of exceptional rainfall,
although the dry bed is clear to see across the fields approaching the source.
Water more regularly flows from Lydd Well, just to the NW of the A429 and NNE of Kemble.
The Inscription reads:-
"The Conservators of the river Thames 1857-1974, this stone was placed here to mark the source of the river Thames"

click to visit Phil Cooks site for full size image
Thameshead in exceptionally wet conditions - photo courtesy P.Cook (c).
(This footbridge has been high and dry on our visits).