Are you talking about using your own sequence of numbers, rather than starting at 0001?
If so, got to Settings>General Settings>Invoice Settings. Click on "Default Settings", and you will find that you can change your invoice number sequence (ie: you might want a different or no prefix, and you might want to start your numbering at 120001
Many thanks for the response, however the settings you are referring to still automatically give the invoice number for repeating invoices whereas I would like to control these with our own sequence.
I've now been told by Xero that this isnt possible, but thank you for your response.