Wandering around my local Poundstretcher/Instore today and saw these behind the till with only a small sign on them.
Mototola Motofone F3 GSM800/1800 phone, in black for £6.99. No top up needed.
Comes with a Virgin SIM (Haven't yet checked whether they are locked to Virgin or not)
Edit: The one I have bought appears to be unlocked...!