Back in the 1950s and 60s, when I was at school, the night bus network
just about reached the limits of London but at sparse frequencies and
only serving major arteries.

I've been to Stratford twice recently (6 day cycling and O2 tennis)
and been amazed how the night bus network is now very dense and the
frequency on some routes (25 for example) is advertised to be much the
same as in daylight.

I have been aware, over recent years, how the day time network has
been evolving and, logically, I know I should have been aware that
things were changing at night too.

When did the night network start expanding or has it been constant
since I was at school?