The bird was spotted at lunchtime in a small, private car park where it showed nicely for 45 minutes. It was actively feeding on small insects in a tall Sycamore, often climbing the steep, brick railway embankment to forage out of sight. I love these birds and it's always nice to find one in London. This bird was only a few hundred metres from where I observed a singing male in May this year. Uniformly mousy grey, it was pretty hard to determine much in the field but I think it's possibly a 1st winter male.
Some good info here on this iconic London bird.