Bob-UK's post in Product Image Does Not Maximise was marked as the answer
I think I have managed to find the problem and a workaround:
My shop was trying to load the fancybox graphics from the following folder:
That is the files such as:
This folder, nor the files existed. I checked with the online demo store and it doesn't seem to exist with that either, although the demo store works just fine.
I created that folder and copied the files into it and it now works much better, although there is still something amiss (doesn't close the fancybox image correctly).
If I could find out what, or where is pointing to this non-existent folder then that would be a proper fix.
I won't make this as solved, but if anyone more knowledgeable knows how to fix the source of the issue then please do.
Many Thanks to Vekia for his help.