I've just managed to do as follows:
1. I had to remove any existing joins to the SAL_DEL_ADDR table.
2. I then linked ACCOUNT_REF on SALES_LEDGER to ACCOUNT_REF on SAL_DEL_ADDR
The invoice then showed the notes field from the Delivery Address but, there are some issued:
1. You can have multiple delivery address for a single customer - I've not tested what would happen under those circumstances.
2. To produce one invoice took from less than a second with an unmodified invoice to over 10 seconds with the changes described above...
John