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