At last I have sorted the problem. Removing the trim to access the belt mechanism was done by laying the rear squab down and removing the latch trim on the interior wheel arch. A single self tapper had to be removed, then the latch trim pulled off. The rubber pad in the centre was left behind. To remove the C-pillar interior trim, unscrew the two small securing bolts (8 mm heads) at the rear of the trim which are accessed by folding down the cover over the jack storage in front of the tail gate. Then push outwards on the bead trimming around the edge of the door opening to expose the edge of the C-pillar interior trim. There are two push in clips. one at the top of the panel and the other approximately at one quarter of the distance from the front along the lower edge. A straight gentle but firm pull outwards directly towards the other side of the car now release the trim panel and all will be revealed.
I did not need to remove the seat belt mechanism - the problem was now obvious. It was a 180 degree twist in the belt at its bitter end, which prevented the belt from rolling up evenly and retracting fully. The problem could not be seen or fixed without removing the C-pillar interior trim. A photo showing the two clips is attached.

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