If you check the registration certificate, I think youll find the bike is registered as a PARADE bike not a RACE bike. Check page 190 of the ACU handbook "Classic regulations"

14.5 Special dispensations

1968 Greeves Oulton. 850cc Norton Commando engines ( Group 2 only)
1968 Bultaco TSS. 750 cc Triumph engines (Groups 2&3)

Four cylinder machines built to a good standard, using production engine castings,including replicas similar in appearance to works machines. FOR PARADES ONLY.

Theres more rules in Classic racing than there are worms in my garden. You really should have checked first. Youve paid 4 Grand for a parade bike that you will never be allowed to race in the Manx, Southern 100, Pre TT classic etc.

The Manx is for "Thoroughbred Grand Prix racing motorcycles constructed during (2 periods are given)for the specific purpose of participation in competition during that era.
In addition, short circuit specials, of a type which can be shown to have a connection with the Isle of Man TT and MGP racing, will be considered on their merits. Such special shall have a maximum of 2 cylinders and must have an appearance of a typical racing machine of the era "

Modified CB 500/4's ( which is what this is,) dont fall into any of the above categories. Youve been had, and I thought you were a businessman. I bet you hope your mate Eric doesnt read this, cos he'll laugh his bo--ocks off.

Caveat Emptor ( or summat like that). The buyer beware !!!!!