I am guessing that it is a pretty light fiberglass sidecar. Ideally, you want something with a pretty flat profile. There is an outfit (pun!) in the States called Universal Vintage Tyre that is tied into a similar company in Britain that should be able to set you up. Look them up on the web and give them a shout.
I use a Avon Sidecar Triple Duty on the front, and a Avon Safety Mileage on the rear. I e mailed Avon with loaded weight of the outfit and all 3 tire sizes and they quickly returned my e mail with recommended tire pressures.