Many thanks for that Tiggx. Google comes up with a photo from the Var (where I took mine) of Ascalaphid - Libelloides longicornis which seems to fit perfectly.
Saved my life - or my ego, anyway. 'What's that funny thing?' someone asked. 'Oh! I know that,' said I rashly, 'I'll let you know.' Then couldn't find the book I'd seen it in.
Today got a better photo:
