Dave Polshaw, it would be unlikely that the bike you saw was a Manxman. Not impossible, though.   The Manx was produced in 350cc and 500cc models, and was an out-and-out race machine, with an oversized tank and straight-through pipes, produced mostly for the Isle of Man TT races. They had remarkably good roadholding, of course, which was always a Norton strong point.
In the early days they had to be lubricated with castor oil (which I should think is where Castrol derived its name) and that is the most nostalgic smell in the world for superannuatant petrol-heads.
Even better than steam-coal-and-steam-and hot grease smell of a locomotive.