In the years we've lived here, we've seen about six adders, most of them small, one even smaller having been squashed flat on the road. Most we've seen while walking along well-trodden paths through low undergrowth, when the snakes have probably been lying in wait for prey.
This is the latest, seen yesterday afternoon, and it's unusual in that it's almost completely black. This site, which is full of useful facts about adders, suggests that some adders darken before they shed their skins, but the discussion on another page of the site, here, suggests that melanism in adders may have an advantage in more northerly areas since the dark pigment absorbs heat faster.
The adder was lying beside a path and moved quickly into the undergrowth before I could take a photo - but then it suddenly came back across the path. If I hadn't been hovering, hoping for a picture, and had carried on down the path, I'd have trodden on it. Maybe we'll see it again - it was just up the back of our house.