Great pic of a nice fat female. [the
Adder of course, not your daughter Ha Ha]
I've found
Grass Snakes are nearly always near water as they eat frogs, toads and tadpoles, Possibly small fish too. Why not try laying a board of wood or corrugated felt roofing near a lake,pond or river and check under it regularly. Last year we had a lot of success.
Slow Worms and lizards love to take refuge under it too. Obviously you'd have to put it [or them] somewhere discreet where no one can disturb it or mistake it for litter and bin it! Yeah give it a go,I'd love to find out how you get on.