Lots of strong 30cm cable ties (at least 12, more would be better),
An assistant.
The cable itself is of a fixed length. It is the outer sleeving that changes its' length. Once the auto adjuster is free to move it pushes the sleeving apart, thereby taking up any slack in the cable itself. I think it would be difficult to reset the auto adjuster while it is in situ. As I have a spare cable (adjuster already loosened), I was able to try to reset it.
1, Fit two cable ties around the normal sleeving, one at each side of the auto adjuster unit (bellows included). Do not pull them tight yet, you are going to thread more cable ties through them.

2, To get the necessary length, connect 2 cable ties together. Make up several pairs (at least 4 pairs, the more the better). Thread these pairs through the two cable ties on either side of the auto adjuster assembly, and then close each loop. Make sure each loop is 90 degrees round the auto adjuster from its neighbours. Now tighten the original 2 cable ties from step one.
3, Anchor the cable at the pedal end. If you can pull the clutch end taut and anchor it, do so, otherwise maintain the load.
4, Slide the sleeving from the pedal end towards the clutch end until it stops. This will release the ratcheting system. Now push (NOT pull) the sleeving back towards the pedal end. All being well, once the sleeving hits the stop at the pedal end, the bellows should start collapsing. Keep the pressure on until the bellows have completely collapsed into their transit/fitting position, and then hold the sleeving tight.
5, Get your assistant to tighten up the cable tie loops evenly around the bellows. Once they are all tight, they should hold the bellows closed, and the cable can now be fitted or stored until needed. Once fitted, you just need to snip the ties that go around the cable, and the others should drop free.

The reason for so many cable ties is that the spring within the bellows is both strong and flexible. If it can bend out to the side it will, and the hard work is undone.
I am confident I have got this right, and explained it clearly. I will (when I can) post photos to help clarify what I have done.
EDIT; I did find that the spring pushed the bellows out to the side, causing the ratchet to take hold again. To overcome this, I re-did the job but fitted a cable tie around the point where the bayonet fitting is, and another around the bellows themselves. That seems to have resolved that problem.

Thanks to myglaren for posting the pictures, and to Trevor Worcester for being my assistant. Their help is much appreciated.