A month on, I thought I would provide a 'final update' ....... 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).
Following the protracted exercise, which caused me to expend an obscene (and certainly not cost-effective) amount of time (albeit with eventual 'intellectual satisfaction'), the fridge continues to work fine, usually maintaining a temp of 4-6 degrees.
So, I have certainly re-learned the lesson - that if replacing an 'obviously faulty' component with a brand new one does not sure a problem, one should not automatically assume that the 'brand new replacement' couldn't possibly be 'faulty' (or, at least, as in my case, 'in need of adjustment') !
Thanks again to all those who contributed to the discussion whilst I was pulling my hair out!!
Kind Regards, John