Thanks Kev888 and Wezzel01 - exactly the info that I wanted to know. Very interesting, far longer keg life than I expected even if the beer can/does go down - or 'off' - much quicker when you eventually start drinking the barrel if kept a long while. That is a bit reassuring if I make stuff and then go away for a bit visiting.
But I also take on board the comments on plastic v ss kegs. Wasn't aware that the plastic can allow in oxygen through the plastic, though I am not surprised, thinking about it and the construction of potable water pipes and equipment. So I think the cornelius kegs will be bought sooner than I had thought before I read the posts! Also, the cornelius kegs seem to have a smaller footprint than the king kegs and my space is very limited, anything space saving is good and desirable! I will check them out at the local home brew shop next time in.
My plastic kegs are kept in my shed which obviously has good light levels when I am working in there (I do some model engineering stuff when I can, my shed is a man-cave really!) but otherwise it is subdued light or dark as there is always a blind drawn when I am not in and the temp in there is relatively stable between 50 -58 deg.F unless it is really cold, so cool but not too cool and certainly not warm. May think about a light obscuring jacket for the plastic kegs but.