In reply to Fraser:
Update:
Got a call yesterday from A J Johnstone saying they'd managed to fix the lens and mechanism. It still has a chip out of the edge of the very front ring, but I can one again use screw-in filters and auto-focus. I was very surprised they'd managed to re-straighten the focus ring as it had been forced out of true on one side by about 5-6mm. Happy bunny, despite the £108 bill, which was much cheaper than even a replacement second hand unit.