I recently saw a bright green snake with black diamonds down its back - is this an adder or something else?

Question

The snake was about 60 cms long, and quite slim. This was spotted in a forest in the East of England.

Answer 1

The only snakes indigenous to England are:

Grass Snake (Natrix natrix)

Adder ( Vipera berus)

Smooth Snake (Coronella austriaca)

It's very likely you saw a Grass Snake.  Here are a couple of pictures:  http://www.bbc.co.uk/nature/wildfacts/fa...

Adders are generally wide and yellowish-brown in color.  They do have the trademark "diamond" patterns often but don't fit the other criteria you described.

Smooth snakes are reportedly rare and more of a brownish color.