Thanks again Joe - appreciate your attention and suggestions. This is not something I do very often either 
However. I’ve dug into the topic and now understand more about MBRs and GPTs and the 2 TB limit. I have solved the problem by deleting the 2TB partition to give me a totally unallocated disk, then in Disk Management right-clicked the disk descriptor box (e.g. labelled “Disk 3 Basic 2794.36 GB Online”) and selected “Convert to GPT Disk”. Having done that I was able to create two partitions of equal size with no more than the usual amount of wasted space 
Actually, the link below was one of the signposts on my path to enlightenment, which I may have found elsewhere in this forum (it’s so easy to get lost in browser tabs)
Cheers
Jukebox
