After a great deal of experimenting I managed to deliver a functioning solution, on very much the same premise as my SQL code. You create a calculated column as follows:

=

CALCULATE (

MAXX (

TOPN (

1,

Sales,

Sales[Sales_Value],

0,

Sales[Sales_ID],

0

),

Sales[Sales_ID]

),

RELATEDTABLE (Sales)

)

What this is basically doing is to order the sales table by Sales_Value (descending) and take the top 1 value (all done by TOPN function) - I added Sales_ID to the sort order in case of tied values. This returns a table with one record per customer (the max sales value, because I ordered by Sales_Value descending), so I just have to pull the column I want from the table. I chose to do this using MAXX, though any function that returns a column value would work because there is only one row per client. I can't recall if I needed the RELATED table argument or not (I'm writing this away from my solution) - I think I did because the direction of the join was incorrect.

Any better suggestions?

Stuart