Hello Henry,
what a pitty that this is the only picture.
Anway: the choice is between Middle and Lesser Spotted Woodpecker.
I agree with you:
- your description of a fluttery bird is (certainly) better for a Lesser
- yes, judging size.... But the difference between those two species should be evident during the observation which presumably lasted for more than just a few seconds (because of the fluttery impression) Right or wrong?
- your bird seems to lack any yellowish or orange-cream hue on the rear flanks and the under-tail coverts seems to be whitish
And yes, texture of the underpart feathers and markings there seems good for a 1cy bird.
Therefore I agree with you: its a 1cy Lesser Spotted Woodpecker