I suppose it's kind of pleasing (in a painful way) that nobody has come up with an answer to my problem as yet ie. at least its a decent problem.
Anyway - self-help always goes a long way, and here is the next clue I've unearthed:
The file mentioned above (FOL0077F.PMI) is referenced in my HIERARCH.PM file - here is the line:
0,0,"I11BSX20:26AC:FOL0077F","1CC25143:My mailbox",Name_Unavailable
The doc "Structure of HIERARCH.PM file" tells us what each comma seperated element represents, though it is the last element that stands out: Name_Unavailable.
That must be a default name (I presume) AND NOTE: the lack of quote marks (compared to all similar lines of code)!
0 = folder
0 = default
I11BSX20:26AC:FOL0077F = Unique object ID - the bold element being the filename (which it is)
1CC25143:My mailbox = The ID of this entry's parent (all other top level folders ref this ID)
Name_Unavailable = the displayed folder name (probably should be 'A - Mail Read'), like the two other quoted entries below:
0,0,"QKNXO50D:2CD4:junk","1CC25143:My mailbox","Junk or suspicious mail"
0,0,"81PAANAG:2CD5:main","1CC25143:My mailbox","Main Folder"
*****
So there we have it - has anybody got an idea about what happened to create this situation?
And also - what is safe to do - can I just type in "A - Mail Read" or is there another file that needs changing?