Okay, so you've tried to do it like this:
DoCmd.ApplyFilter "", "[MOT Due] Between DateSerial(Me.cboYear, 9, 1) And DateSerial(Me.cboYear, 9 + 1, 0)"
and/or
DoCmd.ApplyFilter "", "[MOT Due] Between DateSerial(CInt(Me.cboYear), 9, 1) And DateSerial(CInt(Me.cboYear), 9 + 1, 0)"