Roydon Common near King's Lynn is a good place to see adders, Norfolk Wildlife Trust monitor numbers and leave out square sheets of corrugated iron. The snakes like to hide under these as they are nice and warm, heating up in the sun. Just lift them up and the chances are you'll see adders. Also, for lizards and grass snakes, Lakenheath Fen, Suffolk is good. Grass snakes often seen swimming through the middle of Brandon (Suffolk/ Norfolk border) in summer along the Little Ouse River. I had a grass snake under some corrugated iron on the allotment here for a few weeks last summer as well. He'd have a nibble of my finger when I used to pick him up. And frankly who can blame him! He was under there with a spectacularly angry bee for a while, but that's another story.