Very helpful reply. Turns out from your link - and I had no idea - that the promise to swap one fiver for another actually has a use: "Nowadays, the ‘promise t

Very helpful reply. Turns out from your link - and I had no idea - that the promise to swap one fiver for another actually has a use: "Nowadays, the ‘promise to pay’ holds good in perpetuity for the exchange of old series Bank of England notes which have been withdrawn from circulation, as well as mutilated Bank of England notes, provided that certain criteria are met"