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Establish a single space in the OS Code-Point Open data for postcodes
Ordnance Survey supply the postcode data as csv files but the postcode field is set to 7 characters with 2, 1 or zero spaces between the in and out codes. Refer to page 15 of the attached User Guide on how to update this format to a single space postcode.
Most importanly, make sure your new field is a width of '8'.
The expression is:
RTrim$(Left$(Postcode,(Len(Postcode)-3))) + " " + Right$(Postcode,3)