Problem getting a case clause to work

I have the following code but it does nto work, I think it is something to do with the Round and sum functions inside the case clause.

      SELECT    
            Case When @JobsInvoiceVatiD = 1 then
                  Round(TPP.PartsPaidAmount,2)
            Else
                  Round(Sum(TPP.PartsPaidAmount / 1.175),2)
            end  As JobsSubTotal,            
            Round(Sum(TPP.PartsPaidAmount) - Sum(TPP.PartsPaidAmount / 1.175),2) As JobsVat,
            Sum(TPP.PartsPaidAmount) As JobsTotal
      FROM        
            tblPartsType AS TPT INNER JOIN
            tblParts AS TP ON TPT.PartsTypeiD = TP.PartsTypeiD INNER JOIN
            tblJobs AS TJ INNER JOIN
            tblPartsPayment AS TPP ON TJ.JobsiD = TPP.JobsiD ON TP.PartsiD = TPP.PartsiD INNER JOIN
            tblClients AS TC ON TJ.ClientsiD = TC.ClientsiD INNER JOIN
            tblJobsStatus AS TJS ON TJ.JobsStatusiD = TJS.JobsStatusid
      Where
            TJ.jobsid = @jobsid