In the UK bank sort codes take the format 99-99-99 for example,   Excel plays silly buggers with this,  you can get around it by prefixing it with a ‘ which will turn the field to text but really there is a better way by creating a  custom number format.

Simply go to format cells -> Custom and then enter 00-00-00 as the format.   this has the added bonus of preserving the leading number if it is a zero.

Have fun