My method is not to find the snakes, but find the sort of places that an adder loves to bask in.
As a friend of mine did last year around this time when he almost sat on one in a northern glen. It was on a pile of rocks, probably the remains of an old sheiling, when his dog barked a warning. He got a great photo of the adder actually raised and hissing, although it couldn't have been perturbed in the least as it was in the same position an hour later. We wondered if there was something amiss with it but what you say above confirms that as normal behaviour.
It could have been a close run thing all the same, with the road a fair 3 hour walk over the hill and him not as young as he used to be.
Cheers.