The reason you can’t create a new invoice with the same number is due to the audit trail that’s created for HMRC & to prevent accidental deletion
More info (including how to view) can be seen in @Glenn’s post here:
http://community.quickfile.co.uk/t/why-isnt-it-possible-to-permanently-delete-purchase-invoices/5577/2?u=parker1090
And this thread:
http://community.quickfile.co.uk/t/numerical-list-of-sales-invoices/4201/4?u=parker1090