In case anyone else has this problem, here's the fix:
1. Turn off power and unplug from the source.
2. remove R.hand cover. (Silver screw at rear, and black screw at right top corner, plus lever off plastic clip).
3. Remove main PCB by removing five silver screws.
4. Pivot main PCB away from side, and carefully unplug the two white ribbon cables by gently easing them out. (It may also be necessary to unplug the two black multiwire cables to make this operation easier).
5. Insert e.g. a piece of cardboard in between the two legs of the photosensor which is near the top of the board, (Normally, the tang from the lid when closed, will do this), otherwise you'll get a 'cover open' message.
6. Make sure no part of the main board is touching the metal mounting frame, then with only the two white ribbon cables unplugged, plug in power and power up.
7. An error message will come up, press 'OK'.
8. Turn off and unplug mains supply.
9. Replace the two white ribbon cables, and remount the main PCB.
10. Check no cables caught or pinched then replace side cover and power up.
11. Printing a test page should show no errors logged.