The rare coin has been labelled as the ‘holy grail’ by collectors and it’s bound to come up in your grandad’s old stash. And owning one of these could be worth £140,000 at auctionThe rare coin has been labelled as the ‘holy grail’ by collectors and it’s bound to come up in your grandad’s old stash. And owning one of these could be worth £140,000 at auction