A London Coat of Arms

I have long been fascinated by names and icons - there is always a story behind them; and getting the story helps our understanding of the place in which they are found.
I saw this coat of arms on Tower Bridge, London. It marks one of the borders marking the territory of the true 'City' of London - the 'Square Mile' that is the financial centre of the wider district of London. Shown at the centre is the flag and motto: "O Lord, direct us." (Some might say that a number of misguided leading financiers could do with some direction from on high!)

The City of London is overseen by the Lord Mayor of the City of London - an office separate from (and much older than) the Mayor of London.