I bought these Revolution Wheels, around £80 each, I think -
http://www.ttrsracing.com/images/R138A_BLACK_HRweb.jpg
When I finally found the UK company they were very helpful, and knew which wheels were for which mark of Sumo - of course I haven't actually fitted them yet, owing to problems sourcing tyres...
Here's the address-
REVOLUTION WHEELS
FAYGATE BUSINESS PARK
HORSHAM
RH124DN
012 9385 1776