Snake was sunning itself in a field - spotted it by accident and managed to get a picture before it disappeared!
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You are right that it is a grass snake. If you add a revised identification and click "get recommended" iSpot will add the scientific name Natrix natrix. This will link your sighting up to other observations of grass snakes on iSpot.
Lat/Lng: 51.1245, -0.3574
OS grid ref: TQ150374