£50 banknotes, but also they're often used as a store of value. and i'm sure that's what many of these notes will be used for. it's quite heavy, and i'm holding a quarter of a million pounds in the bank of england's highest value notes. but while you might not get one of these in your pocket compared with a fiver, a ten or a 20, the whole collection has got something in common, because for the first time, they're all made of plastic, not paper. plastic, or polymer notes, last longer. so will we need so many of them? people ask that all the time. i think making polymer is more challenging, and it's different to paper because it lasts two and a half times. but actually, making cash more efficient is also an important part of keeping cash alive. these old paper £50 notes won't survive. you can't use them in shops from october next year. its replacement, featuring wartime code—breaker alan turing, is packed with security features, just like the place where it's being made. kevin peachey, bbc news.