Andrew, as CCM engines got bigger and more powerful the old 250 BSA based gearbox suffered. CCM got Quaife Engineering,a well known Pommy gearbox manufacturer, to design and make a bulletproof 3 speed. The gears are as big as that in a top loader Ford and because the CCM is so torquey, a 3 speed is all that's needed. Unfortunately I reckon that bike would be out of my limited price range.
By the look it's got Simons forks as well. It could well be a 10k bike.