Sounds like a cricket, although the ones on my allotment are also brown. I'm sure they do come in green though.
I stopped to look at one properly on Saturday. Very carefully caught it and picked it up, and then, when I was inspecting it (purely out of curiosity), it leapt straight up, and landed in my hair.
Cue comic sight of Arch leaping about, frantically shaking head and combing hair with fingers...
