bird pecking wood
NOT a woodpecker
intermittent tap, tap, tapping
in a hollow on a willow tree trunk - guess may've been after beetles/grubs, but went on for while (I had to leave to continue run!) so seemed to be enlarging the hole rather than foraging
dark plumage, starling size, quite "squat" though
tinterweb suggests nuthatch, but didn't look like that either
any ideas?