See http://www.gmayor.com/formatting_word_fields.htm
You may need to trap the field error with a conditional field eg
{IF {Mergefield Postcode} <> "0" "{Mergefield Postcode}"}

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Graham Mayor - Word MVP
My web site www.gmayor.com
Word MVP web site http://word.mvps.org
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> I have a client who has a mail merge referring to an
> excel list. The list has postcodes that are not
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>
> Guy