This morning its attraction to the garden was revealed. Feeding in a shady flower border the photo's show that it was eating the seed-heads of dog-violets.
The RSPB Handbook of British Birds (https://www.bloomsbury.com/uk/rspb-handbook-of-british-birds-9781408110577/) states a "rapid decline of over 40% over the last 25 years especially in the south of England" How very pleasing then that this one chose to feed in my garden.
Then, in the evenning of 22nd June 2018, the male is still around and is joined by a female -- both feeding on Amelanchier canadensis berries.
The implications of allowing a formal garden to drift into a nature reserve are just delightful for any nature lover. Please see also http://www.peterlovetttalks.co.uk/page19.html