First, I assume you've all met all the oodles of restrictions for an indexed view; you might want to verify that using Books Online "CREATE INDEX", "Remarks", "Indexed Views" as a guied.

Also, GROUP BY in the base query in exactly the same order as the clus index will be:

create view dbo.withClusteredIdx

with schemabinding

as

select c.Reference, mc.MPAN,ElementID,MONTH(hhdatetime) mm,YEAR(hhdatetime) yy

...

group by YEAR(hhdatetime),MONTH(hhdatetime),c.Reference, mc.MPAN,ElementID

CREATE UNIQUE CLUSTERED INDEX [idxTest] ON [dbo].[withClusteredIdx]

(

[yy] ASC,

[mm] ASC,

[Reference] ASC,

[MPAN] ASC,

[ElementID] ASC

)

go

SELECT ...

FROM dbo.withClusteredIdx

WHERE

yy >= @YEAR ... AND

mm ...