So it seems like the standard error (no more fonts can be applied to this
workbook). However this occurs when I try and make part of a cell comment
text bold, or when I try and save the spreadsheet for the first time (in this
case I get a stack of 'no more font' dialogue boxes before it does actually
save.

There ARE lots of embedded charts in the workbook, but they all have
ChartArea.AutoScaleFont = False (along with anything else I un-AutoScale,
such as text boxes and the like).

I have discovered several interesting things:
(a) I can add new charts without a problem;
(b) that the error upon saving can be removed if I save the workbook about
half-way through the task (which is a pretty intensive VBA program);
(c) if I save and then reload I can change the font to bold no problem.

Any clues on this little foible would be appreciated. The fix whereby I save
and close the sheet after each step seems somewhat clunky...

Cheers,

Matt