Hi Martin, thanks for the reply.... so, I checked the rudder servo in the sink, it appears to be moving right up to the port/starboard stops, no visual difference in rudder deflection from right to left, although the servo does still try to keep pushing when the control is moved to full starboard steering, although that stops if I trim slightly to port. I have two holes in the rudder connector, and it doesn't appear to make any difference which hole I put the wire connector pole in. I also noticed that the servo tab has 3 holes (vertically), but I didn't experiment there as the wire pole looks securely fitted, and would need unbending to get it out....
Regarding the receiver, there appears no water ingress, indeed, hardly any water gets into the boat after a run, it is quite watertight.
I should also mention that the controller had a completely new battery, so it can be ruled out....
Any further thoughts?