Using the following formula:
=IF(ISERROR((BB25-AB25)/2),"",BB25-AB25)/2

But it comes up with #value! errors (some data not available in each BB or
AB column)
I can hide these with conditional formatting but I am then using the
following formula linked to that column of cells:

=SUMPRODUCT(($H$10:$H$89<2)*($I$10:$I$890)*($E$10 :$E$89="Female")($BE$10:$BE$89))/SUMPRODUCT(($H$10:$H$89<2)*($I$10:$I$890)*($E$10: $E$89="Female"))

(The column witht the #VALUE! errors in is the BE10:BE89).

Any suggestions?? There must be a way of ignoring the error cells and just
extracting the data from the completed BE10:BE89 cells. OR do I have to do
something with BE10:BE89 cells in the first place to stop the #VALUE! errors
initially?

Many thanks you clever guys !!!!!