a late flowering, hard growing (almost de rigeur on bare chalk slopes),
white Ground Ivy, Glechoma
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This identity had occurred to me, but the leaves were not quite right! I could not think of a better alternative though. AH.
There a major snags though, as the flowers are the wrong colour and the flowering season is normally March to May.
Notes: Ground Ivy occurs on the chalk downs lower slopes of Mill Hill and is prevalent (very frequent to common) (but not dominant) and the usual month when it is notcied is May. It is too common to be newsworthy, but is recorded for its part in the ecology and it appears to be visited by a fair number of flying insects, notably the Grizzled Skipper.
inclined to think it is a blind spot on my part and it is Wild
The leaves are slighty wrong though. Not toothed enough and not hairy?