Brian hi, edited the query as follows

With Member [Measures].[Measure1] As [Measures].[PI002 Rent Collection Ratio RAT (%)]

Member [Measures].[Measure2] As [Measures].[Value Rent Period Debit]

Member [Measures].[Measure3] As [Measures].[Value Rent Period Receipts]

Member [Measures].[Measure4] As (([Measures].[Value Rent Period Receipts]*-1)/[Measures].[Value Rent Period Debit])

Select {[Measures].[Measure1]

,[Measures].[Measure2]

,[Measures].[Measure3]

,[Measures].[Measure4]

} On Columns

,(StrToMember(@FirstDate,Constrained).Lag(6):StrToMember(@FirstDate,Constrained)) On Rows

FROM ( SELECT ( STRTOSET(@TenancyTenure, CONSTRAINED) ) ON COLUMNS

FROM [CBIHousing])

Where(

[Property V].[Reporting Rent Group].[SomeHA]

,

Iif(StrToSet(@TenancyTenure, Constrained).Count = 1, StrToSet(@TenancyTenure,Constrained), [Tenancy].[Tenancy Tenure].currentmember)

)

;

With exactly the same result "Query(11,18) Parser: The syntax for 'FirstDate' is incorrect