Thanks Beerpig, you caused me to look at the manual instead of relying on memory. I can't find the locknut adjustment at the back of the clutch rod in the Greenfield parts list, I must have been thinking of another mower. This pic is the E2000 linkage.

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I must recant, aussie. I suggest you check your linkage for any bits that have come loose, worn out, or are stuck or misaligned. If you can't find a fault in the linkage, I think you will need to get in amongst the clutches themselves, following the process used in previous threads. Can you let us know what you find in the linkage inspection, with pictures if possible? If the answer is that you didn't find anything, can you post a picture or two of the clutches? You would be moving the clutch pedal back and forth, and watching what happens: the clutch fork will move sideways and you will be looking mainly at the rear of the fork and the centre of the clutch disks, looking for differences between when you push the pedal for forward, and when you push it for reverse.