I found an answer.

I had to use the 'Merge Join' Data Tranformation in BIDS to ge this to work.

Thanks, but I actually managed to do it using the following. It was so easy in the end:

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select top(@number)

row_number() over (order by count(datedifference) desc)as [Rank],

datedifference,

COUNT(datedifference) as Number

from #search

group by...