Look up your ARG (Aphibian & Reptile Group) on http://www.arguk.org/, or your County Ecologist. They’ll have someone who’ll be willing to pop round and advise you.
RSPCA are a waste of time to be honest – if there’s an Adder knocking about in your garden, its could be in every garden up and down the street until someone takes its head off with a shovel. The ideal would be to relocate it to somewhere more suitable.
I wouldn’t bother DIYing it, and I used to do this kind of stuff for a living. FWIW, if you repeatedly disturb it (and it will be in gardens), chances are it’ll go somewhere quieter. It’s very unlikely that it’s confined to your garden, and if there’s a garden on the street that backs onto some heathland or wasteground, then that’s where it’ll scoot off to.