If the outfit was registered before, I think, 1980, you can have the chair on either side. After that date in has to be on the left. Some sidecar drive outfits were sold until recently by being registered as trikes, which they are not as the legal definition of a trike is that it must have 3 seperate tracks, not 2 as an outfit would have. This is a very grey area of legality and there was much concern in the COC that come MOT time these trike registered outfits would not be passed.
Incidentally, the russians only made sidecar bodies with the 'door' fitted for right hand side. If you have a left hand chair Russian outfit between 1980 and the later Uralmoto bikes, the chair body is fitted to the chair chassis with the door opening on the wrong side - ie facing the engine! When Nevals adapted the sidecar chassis for left they simply chopped the chair chassis, flipped it over the other side and rewelded it .. you can see the welds.
Also, on the Dnepr chair you access the boot through the back of the seat but a Ural chair has a proper opening boot lid .. handy for putting your stove in to cook breakfast!! Also the Russian army sidecars can carry not only a machine gun but also an anti tank rocket!!!!..... I'll get my anorak