I've finally got around to looking at this again. I have a setup like the one elbweb proposes above. To recap, there's a table which holds orders, linked to an order_details table by ID number. This is because each order can consist of multiple lines (so each line is in order_details, linked to a single entry where the order number is allocated in the orders table).

I need my system to print delivery notes. Deliveries may consist of:

1) All items from an order
2) Only some lines from an order
3) Some/all lines from multiple orders.

I need a way in my database to be able to identify which items should are ready for delivery, and print the delivery note for them. This will involve creating a record in the delivery notes table with the date and delivery note number in it, then entering the order_detail_ID for each item into a delivery_details table with the delivery note number alongside it. Once that's done, a delivery note report can be run to create and print the note with all the required items on it. I hope this makes sense so far - it's difficult to explain, so please let me know if you're not following me!