I have been searching in the groups for an answer but can't seem to find one
that fits my problem... so apologies if you have said this a few times before.

I have the following formula to calculate the unique records where items
match a specific criteria - but it takes forever to run (and near crashes
everytime!) - does anyone know of anything which would make this less memory
'hungry'? The spreadsheet is 40,000 rows.

={SUM(IF('Client data'!$AN$2:$AN$31533="Less than £100m",IF('Client
data'!$AI$2:$AI$31533="AUSTRIA",1/COUNTIF($B$2:B$31533,$B$2:B$31533),0),0))}

I've attempted a sumproduct - but it has been returning an error so I'm lost!
thanks in advance