Found this article in the 'Manchester Evening News' of Thursday 15th February 1917:
GREAT DEMONSTRATION IN LONDON.
There was a huge War Loan demonstration in Trafalgar Square, London, at one o'clock to-day, after a procession of municipal dignitaries from the Mansion House, headed by the band of the Welsh Guards. In the Square massed choirs, assisted by many professional singers, and accompanied by the bands of the Coldstream and Welsh Guards, led the singing of "O God Our Help in Ages Past."
The article went on to state that speeches were made by the Lord Mayor of London and Mr. Massey, Prime Minister of New Zealand. This find confirms the above post, and the exact date of the film: 1pm, Thursday 15th February 1917.