It's all working ok now though. Looks like I was being a noob and
making a silly error (though it wasn't an obvious one to me).
Thought I'd share since it's possible a future newbie might encounter
the same problem.

anyway...

What I was doing (wrong) was (not indicated in first post):

-------------------------------------
sub_x
<snip>

y = 1
Do While y <= NumberOfSheets


wb.Worksheets(y).Activate
lastrow = wb.Sheets(y).Cells.SpecialCells(xlCellTypeLastCell).row

row = 3
Do While row <= LastRow
acctNo = Cells(row, 2).Value
CoName = Cells(row, 1).Value
acctMan = Cells(row, 3).Value
revenue = Cells(row, 16).Value


<snip>
Loop
y = y + 1
<snip>
End Sub
-----------------------------------------------------------

Now for each sheet I was recording the name, so I know that the y = x
iteration was working correctly because I had a record of the name of
each worksheet... but the data  was the same in each case...

where I was going wrong  was in not fully declaring the location of
each cell (I was assuming that "cells(r,c)" would refer to that cell
in the active worksheet... but it didn't.

When I changed the above part of the code  (shown below) then it
worked fine (ie adding a "wb.Worksheets(y)." before each "cells()"
reference.

-------------------------------------------------

sub_x
<snip>

y = 1
Do While y <= NumberOfSheets


wb.Worksheets(y).Activate
lastrow = wb.Sheets(y).Cells.SpecialCells(xlCellTypeLastCell).row

row = 3
Do While row <= LastRow
acctNo = wb.Worksheets(y).Cells(row, 2).Value
CoName = wb.Worksheets(y).Cells(row, 1).Value
acctMan = wb.Worksheets(y).Cells(row, 3).Value
revenue = wb.Worksheets(y).Cells(row, 16).Value


<snip>
Loop
y = y + 1
<snip>
End Sub
--------------------------------------------------------------