Ok, apologies for the double post. I knew I'd seen this somewhere recently though!
It's the Willis-BMW Special, built by Ron Willis in 1948 on a BMW Type 40 base with an incredibly lightweight body and frame made from magnesium alloy. The wheels, brake drums and sump were also alloy. In 1950 it gained a 328 engine and in 1952 it set a sports car record at Prescott with this motor, but the car was sold by Willis the same year.