Rambling Rob recently posted a photograph of a beautifully coloured female adder (click here) which reminded me of the occcasion when my daughter nearly rode over this adder while cycling in the College Valley in the Cheviot hills. She thought it was a stick but when it suddenly moved she jammed on the brakes and managed to stop just in time. She got such a shock she adder viper brow. (groan...sorry).
The colour variation betweeen males and females is well documented (try here) but I also wonder if there is a geographical variation too? I admit that I have not seen that many adders but all that I have seen in Northumberland have been much darker than Rob's example.
Postscript: A monograph in the Shire Natural History series on the adder (by Peter Stafford) explains that infant and juvenile adders are much more brightly coloured and, particularly in females, can be a quite bright, orange or even reddish shade. In fact they are so differently coloured that at one time they were thought to be a different species - the dwarf red adder. It takes about 4 years to attain the final adult colouration and so there will be quite a broad variation in any population. Furthermore, the dark markings are very variable too. In some individuals the zig zag can even be replaced by a perfectly straight line of marks broken up into blotches.