Thanks Trevor,
I am sure that the fact that I have the rudder up and on a swinging mooring is the cause of the problem. Alas I dare not leave it down. I will try replacing the wipstaff lashing with a heavy duty shock cord as a temporary measure. This will reudce the strain on the cables but likely increase the amount of movement. My intention is to replace the cable with dyneema but need to source and make up the cable replacement in the fullness of time.
Aye
Greg