Nigel,

Press Ctrl + End in the worksheet, this would take you to the Last Used Row.
Though you may not see any data content there, possibilities are that
something was typed there and removed. This is a behaviour we've come across
when users press ctrl + downarrow and accidently type content in the 65536th
row and delete the data but end up with a large file size.

check the formats of the last row and also see if there are any space
entered in a cell.

or just delete the last row.

Arun