I would normally be thrilled to see a grass snake this closely, sadly this one had stopped working and started smelling.
There was a suspicious bulge half-way along, presumably its last meal. At Begwary Brook today.
P.S. I reported this to Helen Muir-Howie and she wondered if it had eaten either a Common Toad or a Great Crested Newt, either of which apparently can kill a grass snake.