How to add leading zeros please

Hi All (especially Rory )
I have a file sent to me with part numbers in and prices however the lower sequenced part numbers that should have a leading zero have lost the zero prior to sending to me. They should all have 5 digits e.g 06766 but this appears as 6766. All part numbers starting with a character other than zero appear perfect e.g 12121 appears fine.How do I get the ones with the missing leading zero to get the zeros back please?

Hi Søren,
thanks for the post. Excuse me but I cannot get it to work. I have replaced "reference" with A2 and get a message coming up saying a problem with the formula. I have previously used a method of formatting the row I think. Does that ring any bells please?

To keep the numbers as numbers but show a leading zero apply a custom format using 0####

Thanks to all. Sorted