Query  ?
 What is this Flower?

I suspect a late flowering, hard growing (almost de rigeur on bare chalk slopes), white Ground Ivy, Glechoma hederacea.
Message on UK Botany (Link)

Comment by John Peacock on UK Botany (Yahoo Group)

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.

Ground Ivy from Mill Hill

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

I am inclined to think it is a blind spot on my part and it is Wild Basil, Clinopodium vulgare.
The leaves are slighty wrong though. Not toothed enough and not hairy?

Wild Basil

Viper's Bugloss

               Viper's Bugloss