Spent some time in the service department at my local dealer today and had a good look through the service manual. My tractor does have a two stage clutch as 8x56mn described above. I suspect that it is not properly adjusted. I've got the instructions on how to do this and am hoping to do it tomorrow night.
We've now reassembled the mid-PTO housing and put it back on the machine. We're not going to test it until we sort out the clutch, to prevent further damage!