From my limited experience, try Googling Portwood Insurance and follow links to their Classic Motorcycle policy.
Alternatively try calling 2gether Insurance, a broker in Wisbech specialising in 'unusual' vehicle insurance.
For under 2000 miles and less than 600 cc I'd expect something like £60-70 fully comp, accruing no 'no claims' bonus and for a rider with another vehicle as main transport. The premium varies a lot according to the market value of the machine.
For my A10 the premium is £88, 2000 miles, nr. Slough, garaged at home. No previous bike no claims bonus but a club member. The bike is a fresh rebuild and valued accordingly.