I also tried
=IIF(Format(Parameters!Start_Date.Value, "MMM") = Format(Parameters!End_Date, "MMM"), IIF(Format(Parameters!Start_Date.Value, "yyyy") = Format(Parameters!End_Date, "yyyy"), Globals!ReportName & " from " & Format(Parameters!Start_Date.Value, "dd") & " - " & Format(Parameters!End_Date.Value, "dd MMM yyyy"), ""), "")
But I'm still not getting it right.