Sorted it - It was a lower level permissions issue, not a higher level. Each of 3 items in the LR folder needed to be OKed, even though the folder they were in was fine. Thankfully it was just the one file for the previews. not the several thousand on the PC.
Permissions should cascade down. :x
Now I have my catalogue open on my other machine I discover none of the preferences and tweaks made it across. Aargh! Though the keywords did!?
LR works differently on a PC and a MAC regarding dual monitors. On a PC it fills the whole desktop on both full screen modes, whereas on a Mac it only fills a single monitor, which means a cramped view, when using either of the incorectly named 'full' screen modes, my prefered way of working, as I dislike unnecessary clutter.
I also use Jeffrey Friedl's Lightroom modifier to allow me to have panels as big as I want so the right hand panel fills the right screen. This has two benefits - images are not cut in half and the sliders can be finessed far more easily.
Slideshow does not display on one screen on a PC, It displays across both. Unlike a Mac where it works properly, on one screen only. Annoying, but less of an issue as the Mac non full screen annoyance, as if I was doing a slideshow, it would invariably be on a laptop [so single screen]. Still a bit beta like in operation.