Many thanks for the help, Gary's Student.

However, I'm not sure what you're saying is correct. Have another look at my
original post. After saving the file in Excel, I then do what you've
suggested (see my step 6 - which should be step 5! sorry): open the CSV file
in a text editor. And the zeros are gone.

In other words, it's not a presentation issue - Excel really has stripped
out the leading zeros. Despite the fact that I didn't go anywhere near the
relevant part of the file (namely, cell A1), during my editing.

All the best
MattM