There were a number of references to The Atom in Wimbledon programmes towards the end of the 1955 season, as well in May 1956. Also, there were several features and references to it in the Speedway News in 1955, confirming that the initial testing was done at Wimbledon, with the car driven by Ronnie Moore. I guess that Cyril Brine took over as test driver after the July crash at Plough Lane in which Ronnie suffered a broken collarbone.
As there are still leaves on the trees in the clip it is likely that Cyril Brine was filmed in late summer, during the period Ronnie was recuperating. The lighting is very similar to that evident in the 1960 Rayleigh team picture on the Defunct Speedway Tracks site http://www.defunctspeedway.co.uk/ but was this photo taken at Rayleigh? The front cover of the Rayleigh speedway programmes from that era shows a photo in which a stand looks similar to that in the film clip.