From a series of Tweets.
This fits with other information that is out there and seems legit to me.
A Location Manager at Pinewood Studios has advised offices in Westminster about B24 Films' plans to film along Thames.
The filming sequence is set at night, and involves an "Action" helicopter taking off from Riverside Walk Gardens, next to Vauxhall Bridge, and initially hovering over the river at approx 150 feet. Following this, the helicopter turns and travels down river at the same height over Lambeth Bridge towards Westminster Bridge. During the filming the helicopter will be followed by a 2nd "Camera" helicopter. Dates to follow nearer the time of each event.
The Location Manager states that the team appreciate that they are very privileged to film in London, and they do not Take for granted the support they are shown from residents and organisations, for which they are hugely grateful.
Also. From a different Twitter account.
Confirmed London #SPECTRE filming: 17th May & 14th June 9pm-12am. Low flying 'stunt' helicopter down the river Thames towards Westminster.
So it seems that this is taking place this Sunday.