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Aside from the location, the protruding stamens suggested E. vagans (the stamens don't project in E. tetralix) and E. tetralix has tighter, more rounded clusters of flowers that sit at the end of its stems.
my book didn't have many options for heather - I should have thought of Cornish Heath!
Lat/Lng: 49.97617, -5.23308
OS grid ref: SW682134