Paul McCartney wrote a song about it...

"Blackbird singing in the dead of night"

If it is a tuneful trilling then it's a blackbird. Nightingales don't like the city and it is probably several thousand years since one sang on the geographical spot that is now Berkeley Square.

It could be a territorial robin doing as Gnu said above, showing off for a prospective mate - and as we all know it's the early bird that catches the worm.

LTS