Buy a Sim Card from ASDA for 49 Pence and insert that.
If it works, great, if it doesn't try and get the phone unlocked from the original supplier using the Internet sites or the local Market stall.
Asda = 8p per minute calls, 4p per text message.
I agree with everything that Wak says but maybe in a slightly different order. I found an old mobile in a drawer but the battery was dead and wouldn't respond to the charger. I got a new battery for 99p from www.mymemory.co.uk and when this responded to my charger I then went to ASDA and got one of their sim cards (free with �5 top up). I now have a working mobile for next to nowt.