Well the heater fan switch arrived today, and being unable to remove the control unit from the dash I once again got into the contortionist routine and endeavoured to screw the new switch into place ...
five hours later I secured the first of the two 1/4 AF screws!
As the screws are under an a/c duct (that I can't get out) with about 3/8" clearance, it is impossible to hold the screw in place with your finger. So I devised a cunning scheme to hold a screw in the ring spanner with a flat magnet ... FAIL! I couldn't get enough turn on the screw to engage it with the thread and therefore lift the magnetised spanner off for a new turn.
Next, I used sticky tape to hold the screws in position on the switch and placed it over the threaded holes (which are unsighted due to their position). This worked AFTER THREE HOURS on the first one. After another hour, the second one refused to start. So being in a good frame of mind by this stage I used the professional approach and threw the efn thing out the door and made sure the first and only one is extra tight.
I have an idea that my methods won't ever be plagiarised by any technical writers.