Well, Aston didn't build very many single seaters, so this was easy to find with a little searching...
It's the 1939 Aston Martin 2-litre 'Brooklands' Speed Model, as modified by Gordon Garside in around 1948. Here's a couple of paragraphs taken from a website that sum up the history of the car quite well:
"In 1936 after Bertelli had resigned, Claude Hill became chief designer at Aston Martin Ltd. He realised that very soon the company would need to develop a new engine for the next series of production. So, Aston Martin established an arrangement with the Cross Company Ltd. to develop an application of their rotary valve arrangement for a motor car engine.
This unique Speed Model racing car was built by the works prototype department, with narrowed Speed model frame and narrower front and rear track, to run at Brooklands with the ‘new’ engine fitted. It was hoped that success would promote the new Aston Martin/Cross engine. Sadly war was to break out and development ceased, but the car did run at Brooklands with a crude single-seater body and with wings fitted, being tested by Gordon Sutherland and Charles Brackenbury, but, with the 1936 ex. Dick Seaman T.T. engine installed. It was found to be capable of 60mph in first gear.
After the war the car was sold to David Brown as part of the assets of the pre-war company, and the car quickly passed into the hands of well-known special builder Gordon Garside. He rebuilt the car with two-seater sports bodywork and raced it in national events, including with the AMOC, in the early 1950’s."
