Hi Paul, I work at Pobjoy Mint. www.pobjoy.com . Although we do not value coins we can give information regarding our coins and how and where to get them valued. firstly, if you did not have the certificate (which you do) you would be able to tell if it was a Pobjoy Mint coin by searching on either the reverse (if obverse contains the queens head) or obverse if it contains a coat of arms for a small descreet 'pm' park, this obviously stands for Pobjoy Mint and we put this on all our coins as part of security for fradulant coins and for people to recognise the coin as a Pobjoy Mint Coin. Now, as your coin is an Isle of Man 1980 it is without doubt a Pobjoy Coin. (Pobjoy Mint have been minting Isle of Man coins since the 1970's). If the issue limit is 1000 then the coin in theory should be worth more, but if the limit is 10,000 it might not be as valuable. That 'piece of paper' as you said is actually quite valuable to the coin, it is not just a piece of paper but a certificate that certifies the precious metal, the fact that it is a legal tender coin and has been approved by the government as well as buckingham Palace and the genuinety of the coin. The Proof 3 means that the coin has been struck 3 times to achieve a superlative mirror finish. My other advice to you at the moment is, the price of gold is rising and rising once in some cases twice a day, it is the highest its been for over 10 years, maybe even longer. If you are intrested in selling this coin then it is worth getting a valuation from a coin dealer. Have a look at Coin news publication, which can be found in any good WH Smiths stores. I know this post is not in conjunction with the date of the original post but i hope it can be of help to you and possibly others. Thanks Pobjoy Mint Team