This is a mature female grass snake (Natrix natrix), she was approx 120cm, or 4' long and quite bulky.
Grass snakes are non-venomous, but they do have a very unique way of defending themselves.
If threatened or attacked, the grass snake will roll over on its back and feign death. It will gape its mouth and the tongue will hang out. If this fails they will then squirt a liquid from their anal gland, which comprises of partly digested food and anal secretions!
I have been unfortunate to have smelt this in the past and it is one smell I never want to smell again!
It is quite difficult to sex these snakes as they have no colour variations like the adder, but as the females grow considerably larger than the males, this is one way.
The other more complicated way of telling males from females is to count the
sub-caudal scales, males have 68-72 and females have 52-56.
Of course you have to get close enough to do this and as the grass snake is "very" shy and elusive, it can prove very difficult!