This is a photograph of a basking female grass snake. If you don't know what to look for you would have walked right past her. She has tucked herself behind a rock for extra camouflage and also for a bit of extra protection.
Sometimes snakes will stretch out to bask, this is when they are cold and need to warm up quickly. Once they have warmed up, they will then coil to regulate how much heat they need to absorb. I will address this later.
You will notice that her eye is slightly blue and clouded. This is also something I will address at a later date.
She was approx 120cm in length and quite bulky. A mature snake and grass snakes of this size and bigger are seldom seen.