Just to complete the story, the steering box was sent to Terry at GEE Ltd last Tuesday and arrived back at The Workshop, Finchdean (thanks Ian W for the recommendation) on Monday having been rebushed.
The car now drives much better than it has done at anytime since I bought it = £400 well spent, I think.
Incidentally, Mark Williams was saying that he had three Morgans on site at The Workshop on Wednesday - becoming something of a resource locally where official dealers are thin on the ground.
Mark