One of my dogs was bitten by an adder almost exactly a year ago, we almost lost him, the vet said we got him there just in time, he was on a drip for 5 hours and as he was bitten on his lip, his head swelled to twice it's normal size, the worry was that his airways would be compromised, fortunately he lived to tell the tale, I was told that anti-venom can only be administered once in a dogs lifetime so I am completely paranoid now, I almost pray for it to be cold or to rain, at least while I am out with the dogs, my dogs are not let off leads when it is really hot, my husband takes them out at 5am before he goes to work and they have their loose running time then and again in the cool of the evening.
For goodness sake be careful trying to move adders as they can put a human in hospital and always wear wellies while out walking in adder territory.