Thanks to peesian for an accurate diagnosis. I tried checking the interior light wire at the interior light itself, but had no clue that the wire to it ran behind the dashboard on the side opposite where the offending fuse was.
Well beyond my capability.
The only positive was that my local Ford dealer (Burton on Trent) have a fixed rate for the initial checking of an electrical fault (£99) which they try not to exceed. Apparently this rate was introduced because of diagnosis work taking so long sometimes and costing lots. Unfortunately for me once they had determined which wire it was, there was still quite a bit of dashboard removal etc to fix it.
Stephen James
January 2011