I didn't know that book existed. I have Guide des Sauterelles. I'd be interested to know how good it is. I am also using a couple of websites and have the Greek and Slovenian books. With nymphs I generally presume they look like adults but lacking wings but that may be a mistake. This one is an adult, looks like it should be obvious and I'm still struggling.
Hi Steve, I have the Guide des Sauterelles but I bought the Sardet, Roesti & Braud book after it was recommended to me by a Spaniard (in https://www.facebook.com/groups/ortopterosibericos) to help with IDing spanish orthoptera. It is an excellent book with lots of keys with drawings as well as photos.
Quite often features of the adult wing are used as part of the identification so a nymph presents problems there. The pronotum is also used a lot and that can change shape through the nymphal stages becoming like the adult in the last two instars - not too bad in the UK where we have so few species it doesn't really cause a problem.
Looking at your adult pic it looks like a (rather plain) female Mottled Grasshopper (Myrmeleotettix maculatus) with the slightly flattened clubbed antennae (which are not always obvious), short strongly-indented pronotum and round-looking head. But if I've 'read' the antennae wrong then I wouldn't know where to look next!
A plain female Mottled Grasshopper fro the New Forest attached.