Identification puzzle

A visitor here the week before last sent in this photograph and asked for help with identifying the plant. She thought it might be a Vaccinium (bilberry) but couldn't decide which. I wasn't certain, either, because the leaves didn't seem to fit what is known to occur here, so I consulted some experts at the Royal Botanic Gardens in Edinburgh who came up with the answer. Rather than reveal what they said straight away, I'll wait to see if anyone would like to have a go at identifying it and then repost the photo and the answer in a few days time.
Malcolm