Does it help if you change this part of the formula...

Interest!$C$6:$C$256

to

Interest!$C$6:$C$256+0

In article ,
"KM01" wrote:

Hi there, thanks for your help. I making progress but havent got there yet.
Niether of your suggestions worked straight off, but parts of the 2nd one
seem to get me almost there. I didnt understand the 1/LOOKUP part of the
your suggestion though.

Im now using:

{=SUMPRODUCT(--(Interest!$A$6:$A$256=E$4),--(Interest!$A$6:$A$256<=E$5),--(IN
DEX(Analysis!$A$5:$G$75,MATCH(VALUE(Interest!$C$6: $C$256),Analysis!$A$5:$A$75,
0),7)=$A$8)*Interest!$F$6:$F$256)}

Having worked out how to use Excel to evaluate formulae (really useful
function!) I can see that the whole array is resolving correctly except the
=index() bit which is evaluating the first row in C6:C256 to TRUE without
looking at the subsequent rows and consequently summing the whole of F6:F256.
How do I get this bit to behave like an array formula?

Thanks again!