Finally got a CM3. Cobbled together a playlist of instructions from all the comments here, but still had issues not addressed by others as well as some issues that have already been mentioned.
In summary: Replaced old CM with CM3. No issues, except I noted rubber feet have moved somewhat since new.
Downloaded CM-Boot-Installer and installed. Downloaded LibreELEC installer and Chewitt Slice3 ,img.gz file from Gordon's links (thanks).
Connected Slice to Windows 7 desktop through USB hub and provided power to it. Got the XBox controller device installed. Deleted driver and reran RPiBoot installer to reinstall driver. Tried to manually change driver. Gave up and ignored XBox driver, assuming it to be some clerical thing, and focus on RPiBoot.
At this point it got messy and blurred. First off RPiBoot wouldn't work. Unable to establish a connection or something. Tried power and USB connection unplug/plug to no avail. Eventually (based on a comment somewhere above) I rebooted the PC. I also bypassed the USB hub and connected directly to PC. So now RPiBoot did something, not sure what, because it was on the screen about 3 milliseconds. Nothing new showed up diskwise. Eventually, after several reboots, Slice power cycles and cable unplug/plugs and rerunning RPiBoot, it seemed to have done what it was supposed to, except I still couldn't see a 4Gb disk. At this point, Windows told me a disk needed formatting. No-one mentioned formatting. What to do? Disk manager showed it was in fact a 4Gb disk, so sure, go ahead Windows, format my disk...
I changed the drive identifier to S: to make sure I wasn't screwing with anything previous. So now I have my 4Gb disk showing up with a name that includes RPi so I assume (correctly as it happens) that I'm over that hurdle.
LibreELEC installer was a little funky to use - basically ignore Step 1, select the Slice3 .img.gz file in step 2, select drive S: in step 3 and hit Write. Couple of minutes and boom, written.
Eject USB drive S: from PC, power off, disconnect USB, hook up to monitor and hey presto, we have LibreELEC (Beta) running. Contents of 2Tb drive still there, so good. Into (Expert) settings, all familiar territory, and set region, time server etc. Temps running at 142F (61C).
All good until I point Files at my NAS folder for movies (over 1000, with another 300 DVDs yet to rip). Starts indexing and suddenly I have this interesting little thermometer icon in the top right of the screen, showing half full. Check temps again, now running at 180F plus (80C plus) and the icon goes to full red. So quick shutdown and power off.
Haven't turned it back on yet, as I am now following the heatsink debate before deciding how to progress...
Thanks for all those that have gone before, and your comments. Not sure I'd have got this far without them! But it's looking good, and very, very, fast.
Yes, I would recommend this upgrade to my friends