I was fumbling around in the shed today (is that a euphemism? no) for a paint brush and there behind the basket I was searching in was a snake - it looked very like a viper to me and was sticking its tongue in and out. I beat a retreat.
Apparently, according to our vet, the only way to definitively tell whether a snake is a viper or not is that vipers have vertical pupils whereas other snakes have horizontal. Not a test I fancy carrying out.