An update, for anyone interested ...
'Touching wood', I think I now, at last, have a properly functioning fridge again. It seems that Murphy's Law was working quite well (never assume trhat unused replacement component are 'OK'!). To summarise the important bottom lines ...
The original thermostat was certainly malfunctioning and I suspect that was the (only) original problem (i.e. my replacing the start 'relay' and 'overload protector' was probably unnecessary). I'm not going to throw it out yet, since it's quite possible that it will respond to 'adjustment', as below.
However, the
replacement thermostat (brand new, albeit 6 years ago, and unused) was drastically out of calibration - with it on its coldest setting, it was opening at 10°-11°. It is adjustable (very sensitive) and when I played around with that (a large adjustment was needed - a number of turns of the screw, not just a 'tweak') I eventually got it to roughly the temperature range I wanted (having, ironically, initially got the fridge 'far too cold'!).
It turns out that the 'interior light issue was a red herring, caused by an error on my part. Although I initially connected the replacement stat in exactly the same way as the original (and have photographic evidence to prove that!), at some point during my playing I somehow managed to get two of the connections (labelled "6" and "4" in diagram below) swapped around. Although this did not effect the thermostatic temperature control, as can be seen, it did ('correctly') result in the light being disabled when the (mis-calibrated) stat turned the compressor off. Below is a slightly modified version of the diagram I posted earlier in the thread, illustrating the situation as it exists in my fridge.
So, if it carries on working, I'm seemingly at long last back to having a working fridge without having suffered any cost (other than the loss of a lot of 'leisure time'
) beyond the investment in spares that I made 6 years ago (and, as above, I think only one of them was actually needed).
Kind Regards, John