Thanks for your reply - I think this is exactly what I need.
I have entered the above in what i think is the code module of a
spreadsheet, with two lists in columns C & D, but it doesn't work - I
guess I must be doing something wrong - I'm afraid it isn't obvious.
Any suggestions as to what I need to do would be much appreciated.

If you double-click on the sheet's icon in the VB Editor, do you see the
code? Does cell A1 have a 1 or 2? Put a breakpoint in the code and
select B1 to check if the code is being executed.

Merjet