Many thanks call paul for a very comprehensive response. I note your comments about MacOS first and Linux later. It’s not that I always prefer to do things back to front, it’s more that these things happen.
Per my original post, I did manage to install Linux and it is working very well indeed. The MBL problem was the demise of my WD My Book Live, which had to be taken offline after a recommendation to do so by Western Digital as the device was at risk to hackers and many users had, in fact, lost their data. I transferred all of the MBL contents (offline) onto the Linux box and did a bit of data reorganising. That data - almost 200Gb - is now backed up from the Linux box onto external USB discs (one using grsync, the other using an app called backintime which is not dissimilar to Time Machine backup).
The latest step was to re-partition the disk, which I did today. As well as a small boot partition and a separate swap partition I have a Linux OS partition of about 230Gb with the OS and all of the above data and another partition, called MacOS, of about 265Gb which is, effectively empty.
The next step is to install MacOS - I have downloaded El Capitan and need to work out how to create an install USB disk from that, which I will use to install on the iMAC. I will try to install onto the box with Linux in place, failing that I will try to install onto a freshly partitioned system and then restore my Linux OS and Linux data backups.
A bit of a tall order, but we are going away for a week shortly, so I’ll take a rest, have a think + read a bit more about it and tackle it again when we return, unless the weather is good, in which case I’ll play a lot of golf first !
Thanks for reading - I hope to provide an update soon but in the meantime any advice & guidance is always welcome.
Thanks, Jack