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The plant is heavily infected with a powdery mildew (most likely Golovinomyces cynoglossi or Erysiphe lycopsidis), making the foliage appear very grey.
There were lots of plants affected like this. Red Dead Nettle in a nearby field was also covered.
Please see my Flickr photo's www.flickr.com/photos/129804972@N07/
Lat/Lng: 53.751251, -1.042655
OS grid ref: SE632287