Many thanks for your prompt and (as ever) correct solution.
Fullywired - thanks to you also. The forumla is more cpmplex than simply adding totals - there are deductions being made in some columns as well. The data is subject to client confidentiality, so I couldn't send the file to anyone. Fortunately the problem's now resolved.
I've been promising myself an update to my limited Excel skills for some time, but I never seem to get a chance. On the other hand......why bother when I have access to such expertise?