Huw.,

I just saw your post again. How long is ID6? You might have some memory
limitations for such a long formula with too long ranges. In the end
you might have to end up breaking your ranges and reapply the formulas
in shorter ranges and then get sums of the interim formulas. I am not
too familiar yet with the limitations Excel has for such long formulas.
If you have some clearer data post again and maybe you will get a
clearer answer.

Regards,
Kostis