There is probably a really simple answer to this but it's getting towards the
end of the day and my brain is tired

='R:\Planning for Risk Continuity Security\Task 1000 NFR MW\[Risk Register
as at 18 10 06 (one MW is amending).xls]Risk By Function'!$AB$20

this reference brings up the value in the relevant cell but.....

='R:\Planning for Risk Continuity Security\Task 1000 NFR MW\[Risk Register
as at 18 10 06 (one MW is amending).xls]Risk By Function'!$AB$107

this one doesn't even though they are identical apart from the cell reference.