Hi Alex ,
My contribution FWIW Assuming that you are running 22"rears, that your Caterham 6 speed is direct top, and that your rev limit is between 8500 - 8750, the 3.92 diff should be spot on for your car (as it is for mine (which as a fellow Vx luddite you are welcome to try) running the Quaife seq 6 speed with direct top ). You will be geared for 130mph + and will never max out in top on a UK circuit. Both Arnie & Mark have run big Vxs in the past with the Caterham 6 speed box, so don't worry about the final drive just get on & enjoy driving your car .
L7 FUN