Thank you very much for you help, this solution works well.

However, I have hit another snag

This calculation needs to be applied to a large table, the a+b formulas
don't change but the cells used in the formulas do increment.

So we have the following scenario:

Row 3 Check column A if this is a fri then check the contents of column
B, if it matches column heading on sheet 2 then perform calculation
using values contained in the column on sheet 2 row 3.

Row 4 Check column A if this is a fri then check the contents of column
B, if it matches column heading on sheet 2 then perform calculation
using values contained in the column on sheet 2 row 4.

And so on and so forth.

Forgive me if I am being stupid but I cannot see a way of using a
Lookup table to increment the cell references in the calculations, so
that they stay in step with the calculations

I hope this makes sense

Once again, thank you for your help

Theresa