Thanks kavadni. I have "solved" the problem by super gluing a 6x3 mm plastic spacer that I had in my spares box and then drilling out the hole to 5 mm. The 52T gear now fits nicely on the shaft. Took it for a quick spin, nothing broke off so should be OK for a race this week.
Below are a few pics of the HPI spur gear adapter with non-Tamiya gear attached compared to a standard Tamiya spur gear and the spur gear adapter fitted to the motor mount that comes with the carbon chassis.