Hi Simmo!
Maybe you can check this video to get some comparison. He drives on some bumby roads and the steering wheel has some movements...but it does not seems to be as bad as you described it. He also mentions that the steering wheel give a lot of feedback about the surface - but not at all in a negative way.
http://www.teknikensvarld.se/2011/09/21/24030/morgan-3-wheeler-provkord/You seems to be experienced in driving vintage vehicles so I think you are right, it sounds as if some suspension angles were a bit wrong...
Should be easy to correct once diagnosed, good luck!
