Westwood T1800 Steering problems?

This old beast does a fine job in the garden, however steering problems result in a horrible furrow if not careful. the wheels are aligned in the straight ahead position, on right turns both wheels follow each other properly around the turn, even at full lock. On left turns however unless very careful the left wheel turns to an almost right angle, whilst the right wheel appears to be in proper position. The ball joints appear fine and if the tie rod was shorter the wheels would not be in alignment in the straight ahead position. Really scratching my head on this one and the misses is none too impressed with the odd semicircular furrow in the lawn!