The message
from Brian Mitchell contains these words:

It's small, dun-coloured, has a very pronounced bobbing or curtseying
motion, and its short tail feathers stick up almost at right-angles to
its back. Its wingbeats seem louder and more staccato than most birds of
its size.

It has made a nest in a vertical earth bank under a fringe of long
overhanging grass and seems not to be at all phased by my proximity. I'm
intrigued to know what kind of bird it is.

Janet Baraclough writes
Flycatcher?

Wren's are really tiny birds, and as you say the tail sticks up and is
very short, almost non-existent. Mine hop about on and under the hebe's
picking up insects.

Flycatchers, IIRC, tend to land on fence posts and then execute an
aerobatic loop to catch a fly / insect, and then land back again on the
same post, and they do this repeatedly.

--
David