I was wondering when this would happen. The last time there was a new note the change over period was quite short, only a few months, and I got caught with about £300 'under the mattress' as it were. Bank changed them with no problems.
I don't keep all my money under the mattress by the way, it would be too near the ceiling ;-)).
Don't worry, most banks will still take them back even after they cease to be legal tender.
After my mum died in 2004 we found around £8k secreted around the house, her emergency stash I suppose, around half was in the type E (Michael Faraday) note which was withdrawn in 2001. My dad gave us £2k each. I took mine to my bank and they accepted them with no problems...