Hi Simon,
If you are using IS 4.2, you can achieve the desired result by defining the view in 2 stages:
1) Create IS view (IS_VIEW_1) to select:
Column 1: "VENDOR_BANKING".BANKN_Bank_Account_Number
Column 2: substr("VENDOR_BANKING".BVTYP_Partner_Bank_Type, 1, 3)
2) Create IS view based on view created above:
Column 1: <IS_VIEW_1>.<Column 1>
Column 2: <IS_VIEW_1>.<Column 2>
Column 3: count(<IS_VIEW_1>.<Column 1>)
GROUP BY <IS_VIEW_1>.<Column 1>, <IS_VIEW_1>.<Column 2>
Regards
Nigel