Just thought I'd mention a superb new book courtesy of Wild Guides who have already produced some fine books on orchids, dragonflies, buterflies, etc:
Britain's Reptiles + Amphibians by Howard Inns
There are excellent species accounts with photos of native + introduced species species as well as the various turtles that have been recorded around our coasts with info on distribution, biology + conservation.
At the beginning are summary ID charts, ID from sloughed skins + of tadpoles.
Also chapters on habitats, conservation + gardening for herptiles. Excellent value for money + a book I'm really enjoying at the moment!