I recently saw the stitching was already wearing through. That wasn't the case, ever, with the plastic; I had melted gullies for the tread into the material. That way the stitching was protected. That worked! In the end it was the plastic breaking that was the problem. And the leather pads don't have sunk stitching so it wears through quickly. I decided I needed to repair the old batch so I could take the new batch out of circulation for repair before things got much worse.
As I still had leather left I decided to remake these old pads in the image of the new ones. Easy come, easy go! But I think as soon as I repair the new pair I should improve on the design; maybe try curving the leather so the knee pad sits nicer around the knee, and maybe try to sink the stitching. That will mean more work, but the balance between effort and result might still be as good as that with the plastic. I was really chuffed with how that worked out, but it is a lot of unpleasant work! Let's see how this pans out. One day I'll find the best way of producing custom-made knee pads... as long as the digs go I will be wearing them out quickly enough to get a lot of practice!
Work in progress