To be pedantic, you can't have sent anything by 'registered post' because that service (per se) hasn't existed for years. (You now have a choice between 'Recorded Delivery' and 'Special Delivery'). However, either way, entering the reference number here should show not only the date and time that your package was received but also the signature of the person who signed for it:
http://track.royalmai...45&keyname=track_home
If the vendor owed you money at that time, they still owe you money now. Hassle!
Chris