Well it came back (the problem) as soon as it got hot again but I think I have nailed it this time.
I noticed on the weekend that whenever the thermofan I added years ago came on the SLS compressor stopped. I also found that the alternator was not always charging. I pulled the blade fuses for the thermofan so I could test the current draw across the fuseholder with a multimeter and found the fuses melted but not blown. At it was late at night I just replaced the fuse. Next day the SLS continued flash its alert and when I pulled the new fuse it was also melted but not blown. As you can see the two melted fuses are almost identical.
Tests showed that the melted fuses were no longer providing a path for the electricity (the swollen nature of the melted fuse stopped the blade on that side connecting with the fuse holder). The resistance across the fuse holder with the fuse removed was between 200 k ohms and 1.3 Mega Ohms!!! The blackened and slightly rusted fuse holder was providing the path to power the thermo fan (without a fuse in place) albeit often at very high resistance and thus very high current draw. I also had a very hot but not yet fused fusible link at the battery.
I think this explains the non charging alternator, flaring dash lights and what I thought was marginal battery. Also as the resistance on the shorting fuseholder varies, sometimes things worked OK (the SLS was OK and the alternator charged).
Essentially I think when the relay (set up to cut the fan in when the air conditioner compressor is on) switched the thermo fan on the current draw was so great the compressor couldn't run. Off course most of the time when I was testing the SLS I turned the air con off so I could hear the SLS compressor running (no thermo fan or air con fan noises).
Well so far replacing the fuse holder and putting a temporary fusible link in (awaiting Ford part on order) seems to have fixed it. However, it was not hot today, and I had a few false cures so we shall wait and see before calling this a victory. In any event it is an interesting fuse fault.