I just issued the below (I've removed some stuff specific to us) to our accounts team and stuck it on all of their screens! Hope it helps someone until fixed:-

• Create the Invoice in the normal way (ie, go to the Estimate, and click "Create Invoice" and choose whether this is a staged payment invoice or a full amount invoice).
• Check that the INVOICE NUMBER is correct and add an "x" to the end of the number. Ie, instead of 3992, make it 3992x
• Do what else you need to do with the Invoice to prepare it
• Now - click SAVE to save the invoice (do not "save and send"), then click MORE and click COPY. An exact clone of the invoice will appear.
• The invoice number will have INCREMENTED by one (so it will be 3993x). Change it back to 3992, removing the "x" from the end.
• This can now be printed, saved, save and sent or whatever in order to issue the invoice.
• When done, go back to the invoice list and open the invoice with the "x" at the end of it. When open, click MORE and DELETE to delete this invoice

This way if you have a bunch of invoices to raise, you know that you just need to always never have any invoices with an "x" after their number in the list.