Problem now solved. Thanks to the both of you for your suggestions and advice because I needed you to tell me that I was looking in the wrong place, as I suspected. It was my fault as although I do have Selling Manager (not Pro), for ease and simplicity I generally use eBay's normal All Selling, and under that there is no provision for printing Packing Slips.

However, using Selling Manager, it was a very simple matter of going to the Actions menu and selecting "Print address labels or invoices" which ultimately takes me to the page where I have the option of printing either the address label or the invoice / packing slip, which is exactly what I wanted. I knew it existed somewhere, but it was my fault for looking in the wrong place.

However, without your combined efforts I probably would have taken forever to finally figure out exactly where it was. Taught me a valuable lesson though; stick to the one method and don't switch between them purely for the sake of convenience, because that's what brought me undone. Thanks again to the both of you. 🙂