I believe it was due to the gas pressure failure in the shock. The seems to have altered the ride height at the LH wheel by about 10mm, which was obviously upsetting the suspension geometry. The steering is definitely straight now, and the vehicle feels much more planted to the road, without the slight oversteer it had when turning right.Interesting that the shocks being out could be detected on the steering wheel.
The vibration has gone too, which I'm sure is down to the broken mass damper on the RH strut.