thanks for the comments and advice folks. I agree about the colour of the bluebells they're too pale and washed out. This is due to the high level of lighting used : sunlight strength 12, plus enviro strength on surfaces 2. This was all because of the lack of light penetrating through the tree canopy. If I'd rendered at default settings for lighting you'd hardly see any detail at all. I think I'll retry with a reduced tree density and thus increase the amount of light hitting the ground. This plus pumping up the bluebell colours and reducing reflectivity should work.
@ yossam, thanks for the link- very useful resource.
@Badger, I like the idea of a path.