The Overton Special

I am writing a history of the hillclimbs that took place at Great Auclum near Reading, Berkshire between 1938 and 1974. As part of my book I am producing short driver profiles and would like some help with a car that was driven by Major Charles Lambton at GA in 1955 and 1956.
The car in question is a 1172cc Ford-engined Overton Special. The car was entered as a sports car.
I have tried various ‘specials’ groups and drawn a blank.
Any help will be much appreciated.