I have a workbook with 4 worksheets which each include a column of
numbers with the string "Old Numbers" (say) as a label at the top of
that column of numbers.

11 rows above is the formula;
=Max(Old Numbers)

I have a number of workbooks based on this useage, all of which
function perfectly.

I have "Accept labels in Forumulas" checked - that is, it is on and
functioning.

In this particular spreadsheet, 2 workbooks evaluate the formula
correctly, 2 do not.

For the 2 istances currently working, if I F2 the cell with the
formula, the range used is outlined in blue, but extends to row 16421
when I only need up to 2941. If I then press escape, the currently
working formula continues to work.

If I F2 a currently working formula and press enter, it stops working
and shows a #NAME? error in the cell, just like the 2 which don't
work. That is how the 2 which don't work came not to work. Usually,
F2, Enter simply changes the outlined range to the first contiguous
group of cells under the label.

Another procedure;
Ctrl, End, then delete every row back to the rows actually in use
Save
Close
Open
F2 a working formula

The outlined range is now only to row 2941, but again, an F2,Enter
renders it erroneous, as above.

Any clues gratefully received.