I have just delved into the Fairthorpe Register, and have found something I didn't know before, namely that early Electron cars had bodies supplied by Microplas. My original doubts that the puzzle car was an Electron were based on the obvious fact that it had a Microplas front end, and what appears to be the rear end of a Fairthorpe Atomota saloon. I have found photo evidence that the Microplas shells supplied to Fairthorpe came in 2 styles, one of which had extended fins at the back.
So I'm pretty sure now that this IS the Electron raced by Brown, especially since the text on the website mentions that his car had been modified - hence the headrest which was not a standard fixture. I assume that the caption was incorrect in stating 1957, when he raced the car from 1958 onwards.
So the case is solved.