I do suppose your 'bank supplied number' is a text string.

You could do this straigh, like below:-

Using Val([fBankNumber])

SELECT tblBankNumbers.fID, tblBankNumbers.fBankNumber, Val([fBankNumber]) AS
Clean
FROM tblBankNumbers
WITH OWNERACCESS OPTION;

Or using a function (witch could be more flexible and give more control if
needed):-

SELECT tblBankNumbers.fID, tblBankNumbers.fBankNumber,
ReturnNum([fBankNumber]) AS Clean
FROM tblBankNumbers
WITH OWNERACCESS OPTION;

Function ReturnNum(stgX As String) As Long
   ReturnNum = Val(stgX)
End Function

Success,
Ludovic