You will always be able to get their value because they contain a declaration signed by the govenor of the Bank of England which promises to pay the bearer £x. You might not be able to use then in a shop but they will always have their value.
bednobs - No, SD is referring to the fact that the paper notes are being withdrawn and may no longer be accepted in shops. They will always be changeable in a bank.
Dave has switched off and gone to sleep under the hedge, Roy. He says that whilst the pound may have slightly recovered against the euro during today it is still 5% down from immediately before the budget and 10% down since its high at the beginning of August.