I suspect it's the atrocious aerodynamics of a Seven that's eating up the fuel. According to
this page, the aerodynamic drag of a Caterham is about 50% worse than that of a typical sedan (although better than a VW van or a Land Rover Defender).
Caterham was able to pull out 131 MPG out of a Seven with a stock K-series engine purely by improving aerodynamics and by careful driving:
http://green.autoblog.com/2006/07/27...aterham-seven/