Yep, that's the one

It went back to Bonneville in 1959 with Phil Hill driving it and managed 254.something mph. I *think* this still stands as a class speed record (that's the link for the last photo - a Mossy Hill). The whole car was built after hours by the engineering team, I recall reading (probably in John Thornley's book Maintaining the Breed) that Syd Enever, the chief engineer, stripped and rebuilt the engine one night not long before leaving, as he was disappointed the engine was only producing 240hp. Result was 300

Another story of Syd Enever and, I think, John Thornley (although it may have been someone else - Syd Cousins perhaps?) spend another long night with a hole saw lightening the whole chassis. Although, it may have been one of the earlier cars, I'm not 100% sure.
And here's the image I borrowed, of the engine in it's current house at Gaydon. The whole story is one of the things that inspired me to become an engineer.
And one of Hill in 1959 with EX181, showing just how small it is.
And alongside EX135 and EX179
http://www.flickr.com/photos/triggerscarstuff/4650567277/lightbox/