These sound like damselflies (Odonata; sub-order Zygoptera). Likely to have been one of two species; The Banded Demoiselle, Calopteryx splendens, or the Beautiful Demoiselle, Calopteryx virgo. I favour Calopteryx virgo from your description. See images here:
C. splendens - http://www.dragonflysoc.org.uk/caspl.html
C. virgo - http://www.dragonflysoc.org.uk/cavir.html
Thanks so much. I'm sure you are correct with your answer, looking at the photos you sent the adult male damselfly does look very much like the insects I saw and the stream where I saw them did indeed have a gravel/rocky bed.
Shame they weren't real fairys, but there you are!