Information from the Royal Mint
http://www.thenewpoundcoin.com/
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The new 12-sided £1 Coin.Arriving in 17 days.The pound won’t be round for much longer.
The legal tender status of the round £1 coin will be withdrawn on 15th October 2017. From this date shops will no longer accept these coins, but you will still be able to take them to your bank. We would encourage you to use your coins or return them to your bank before 15th October.
Information from The Bank of England
What this is basically saying is that the pre plastic five pound notes will not be able to be used in shops etc after the 5th of May this year (that's 8 weeks time) so dig out your hidden hoards and spend any fivers in there
http://edu.bankofengland.co.uk/knowledgebank/what-is-legal-tender/
Click on tab four for the relevent information.
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When do Bank of England notes stop being legal tender?
Bank of England notes stop being legal tender when we announce that they are withdrawn.
Before this happens, a newly designed banknote is produced and enters circulation. Bank of England notes keep their face value for all time. If your local bank, building society or Post Office is not willing to accept them, then you can exchange them for new notes at the Bank of England.
For the most recent note withdrawals, we publicised the withdrawal date three months in advance. For the current paper £5 note, the period is longer: it will be legal tender until 5 May 2017.