I'm bad indeed at bird recognition but I saw one the other day
that I really
do not recall before. It was a very little larger than a
sparrow and much
the same colour but while it was on the ground, its tail had a
constant
'bobbing' action, rather like a wagtail. Can anyone help me
ID this one?

Sounds as if it could be a Dunnock? It used to be called a Hedge
Sparrow, and it looks superficially like a sparrow, though it's
not actually a sparrow but an accentor. They do flick their
tails, though I wouldn't describe it as wagging like a wagtail.