I have now solved the problem, and the solution may be of interest to others. The wheel is raised on a cable and at the fully raised position, a sprung hook arm engages into the lifting yoke. The top end of the sprung hook arm has to be depressed by the jack handle before the winding nut can be engaged. If the cable slackens with age, all the weight of the wheel is then taken by the hook arm, and it then becomes imposssible to depress the top end and get to the nut.
The solution is to put the road jack under the spare wheel and jack it up hard. Then you can disengage the hook arm, jam it back with a screwdriver and proceed to tighten the cable to take the weight of the wheel.
I suggest that you should check your vehicle, 'cos this would be a difficult job by the roadside!
Mike, March 2007