I believe that this issue has now been resolved.
What had been happening showed that the column spacing in one or the other pane, (not both at the same time), was changing more or less at random on different days and some days not at all.
I made further checks and found that theConfiguration > Explorer Panel Settings had, for reasons unknown, changed my long term setting for Autosize columns in Sorting and Columns had changed from unchecked to checked.
Once I reversed this and reset to unchecked the issue seems to have disappeared as my preferred settings are now retained.
My initial confusion was due to the randomness of the changes. I would have expected any "Autosize" option to affect both panes at the same time, especially as the panes were set at the same width, approximately 500 px.
All seems fine now so I'll class the issue as Resolved