According to the Ordnance Survey, this is where the Thames first rises 130 straight-line kilometres west of London. Just to the north is the site either of an Iron Age hill fort or a Roman encampment, the exact nature of which is awaiting an episode of Time Team. Apparently there is a soggy field, a stone to mark the spot and a chance, after a particularly wet winter, that you might see some water. You approach down a minor road that turns to gravel once you’re past the private houses it was built to serve. The line of the river is marked by a dense stand of trees, and the source of the Thames is beyond that.