Once again, thanks to everyone for your replies and suggestions, and here's an update:
To recap: I removed the rudder assembly over the winter and found the bottom tip of the rudder blade was almost 6" too far forward when fully down, so I assumed the nylon stopblock had come off. However, the block was still there, so I fixed another one to it, so as to increase the thickness by a couple of inches. I adjusted the thickness so that it put the rudder into exactly the same position as shown by the illustration in the manual.
I was uncertain what would happen when the boat was launched, but in fact it seems to have fixed the problem. There is now no longer a tendency for the tiller to whip across into full lock when left unattended. However, the boat now turns consistently to port under engine. At low revs there is just a gentle pull, but at high revs it pulls quite hard. I didn't get the chance to try it under sail, as there was no wind.
I'm assuming this is caused by the wash coming from the prop. Do other people have this?
All the best, Peter