Depends a lot on where you are. They are common on heathland, particularly in the West Country. Here in south Cambridgeshire I only see one or two a year, always grass snakes.
I definitely saw an adder some years ago - it was at a place called the Badger's Holt in South Devon (strangely named, because badgers live in a sett and otters in a holt)!
I had 2 grass snakes swimming in my fish pond the summer before last, I couldn't believe what I was seeing, unfortunately they ate all the fish, I have also seen a few Adders this summer, one of which bit one of my dogs almost killing him, he was on a drip for 5 hours at the vets.