Quick Update: (I've not been able to work on this for two days)

This problem is due to User Permissions but I have yet to work out why (it's basically the same stored procedure as the original which works fine).

SQL Profiler returns the same query (calls the same Stored Procedure) using identical parameters whether the report is executed via VS2008 or SSRS 2008 R2.

So I use the extracted T-sql being called and ran the SP under both accounts (mine and the SSRS acct), the SSRS account returns no rows (but mine does).

However, this Stored Procedure has the same permissions (Execute-Grant) assigned to the same role so I'm stumped.

Will post further findings here.