At long last, I managed to get my new RIB in the water to-day, albeit for a gentle potter around Chichester Marina.
I noticed that the o/b appeared to wobble slightly from side to side when trunding along @ 1500rpm. When I got her back on the trailer I found that the bolts holding the rubber bushes at the base of the leg were loose where they pass through the "support tube". The bushes themselves appeared OK. These bushes appear to dampen twisting of the motor in relation to this tube.
I checked on a few other o/b's nearby and they all seem to have these bushes. Before I chat to the local motor man, could someone please tell me what these bushes are known as.