Can you get more information on this? It is unusual to encounter an adder in this weather unless bright and sunny and it would be the first confirmed sighting this year. More information on the envenomated region and health of the dog would be really helpful as the potency of venom is high due to lack of use over the hibernation period. Pictures of the area would be superb too if you can get any.
From my understanding of the events - the dog was digging a burrow on the common then yelped. The owner saw a snake disappearing back down into the burrow. I'm guessing the dog must have disturbed it's hibernation.
In terms of the dog's health. It went very ill very quickly, was rushed to the vets and has recovered well. I'll see if I can get the owner to send me a picture of the bitten area.