Vet here in Cornwall suggests keeping Piriton tablets with you, if you suspect a bite give half a tablet, if possible carry your dog to the car to reduce movement and hence the spreading of the venom and get to a vets.Been a few incidents down here especially on the coast paths, although one last year was on a beach, Adder was in the seaweed, and recently there was an adder on Caerhays beach, dog owner saw it fall from the grass bank on the cliff side.