@arsy96, yes you are correct! This is a Costin Nathan.
"Roger Nathan had made a name for himself in the early sixties racing Lotus Elites and then set up in business developing tuning kits for Imps.
In 1965 he colaborated with Frank Costin in the design and development of an Imp engined sports-racing car.
Frank Costin is the well-known aerodynamacist responsible for the design of many early Lotuses, the Grand Prix Vanwall and the Costin-Amigo. He is the brother of Mike Costin, the 'Cos' in Cosworth and is of course himself the 'Cos' in Marcos.
The Costin-Nathan was based on a monocoque centre-section made of resin-bonded plywood (as is expected from the Marcos designer...) with front and rear frames made from steel tubing.
The Imp engine was mid-mounted and angled in the chassis at 54 degrees to the right - a revised lubrication system being necessary for this installation. The gearbox was an Imp unit with close ratios mounted upside-down. On the prototype the body was an open two seater but later models were coupes with typically Costin rounded aerodynamic bodies.
The cars were raced largely in Britain in 1966 and 1967. In '66 Nathan raced the open car six times and won his class five times and was second once. He also set five new class lap records."