Thanks for the comments.
Multiple email addresses is fine, but we need physical invoicing addresses for the actual PDF. The solution you have used would work on the odd occasion, as you've done, but our customer base has grown significantly over the last 3 years and probably 25% of these have offices and different billing addresses globally. Not the old setup of one central accounts department anymore. Have to 'cheat' the address makes it much more time consuming. Without the correct address we don't get paid - it's that simple, as many of these companies now outsource the accounts payable.
A data set is already being held for the 1st address so adding a new data set to the database is not too hard. It maybe that we are outgrowing the current clearbooks setup, which would be a real shame as we've been in from the start and it's a great system.