Thanks for your response Garry.

I've managed to solve the problem by swapping the methods (see my
reply to myself). However I did try using an automated instance of
Excel but although it passed the CanCheckOut test the code then failed
at the CheckOut method with an error saying the workbook couldn't be
Checked out. Hmmm, that was before I swapped the .Open and .CheckOut
methods though - maybe I should try it again. One less click for the
user if I can get it to work.