Queries :: Incomplete Capture In Query

I have a query which exports to a excel file afterwards, however, it's not capturing all the data I need.My query is at the bottom, and I think the problem is the "(([tbl Master].ID)=[tblRequest].[Cost Centre]))" statement, as this means that it'll never pick up a Cost Centre starting with "Z", as they don't exist in [tbl Master] (they're exceptions to the rule basically).

What I need to do is only parse the statement "(([tbl Master].ID)=[tblRequest].[Cost Centre]))" if the Cost Centre doesn't start with "Z", if it does, then it'll find it in the table, as there's validation on the point of entry that don't start with it.

Code:

SELECT tblRequest.RequestID, 'N/A' AS [Week No], tblRequest.[Refund Date] AS [Date],
tblRequest.Requester, tblRequest.Authoriser, tblRequest.Refunder AS Processor,
tblRequest.[CRIS Reference] AS [CRIS Ref No], tblRequest.[Customer Title] & ' ' & [Customer Name] AS [Customer's Name],

[code]....