This is what I found. The thermostat had the markings- A160 and 4180, nothing else. I assumed (dangerous) that the 160 may be the opening temp in Fahrenheit (=71cent). I placed it in some near boiling water and nothing happened. I placed it in a pan with a thermometer and turned up the gas, as the temp reached 99 deg C it opened a fraction (no more than 1mm) and I noticed that there were signs of leakage of a waxy substance! I considered it to be U/S and sourced a new'un with an opening around 70-75 deg C.
The radiator seemed to be in reasonable condition inside. No worrying amounts of muck or restriction in flow were evident. Upon inspection, the gills of the core were quite considerably blocked, especially the lower third, with insect and road debris. Also quite a lot, maybe 5-10%, of the gills were 'bruised'. First I spent an age straightening out the gills, after which I drove out almost all of the debris by repeated water hose and compressed air from both sides. This took an age as the gills are perforated and are almost designed to trap anything that gets past the outermost edge. It is not possible to poke anything other that a slim pointy thing through them (which does nothing expect further compact any debris!!), anything else catches on the gills and does more harm than good. My patience and persistence were rewarded and I was very happy that the radiator was almost 'as new' once it had a light coat of mat rad paint.
Next, the fan switch which is located in the top hose connection of the radiator. Its 'on' temp was 86 degC and its 'off' temp was 77 degC!! No wonder the fan was running all the time. I didn't even bother testing it. I replaced it with one that goes 'on' at 95degC and 'off' at 88degC.
With it all reassembled, it was time for a test run (I checked there was no air trapped in the top of the rad by cracking the bleeder at the top/front of the rad). Fortunately the weather was pretty hot when I test drove it, and low and behold it performed beautifully. It held a temp very slightly beyond midway between 70 and 100 on the gauge. It held this even when I kept it in 4th for an extended time. At no time did the fan come on. In the driveway I let it tick over until, at about 95 on the gauge, the fan came on. It remained on for maybe 4-5 minutes during which the temperature steadily dropped and the fan stopped, as expected, at about 85 on the gauge. The test run was completed without the intake deflector or the number plate and bumper assy. Whether or not this will make any difference, who knows? I suspect not, now that the cooling system appears to working as it should.
So there we are, all done and I'm very pleased with the outcome.
Many thanks to all who contributed. Glenn