That depends. How long have you had the phone, and how much have you topped it up by? I believe Tesco's policy is if you've had the phone for 90 days or more, and topped up £25 or more, you simply call them and they'll give you the unlock code for free.

Otherwise, it's a little trickier. There are many websites that offer unlock codes for cheaper than the £25 Tesco will charge you otherwise, but you must find a reputable one. I have never unlocked a phone, so I don't know what sites are or aren't reputable.

EDIT: And don't bump your own topic after 16 minutes.

EDIT: You may want to read here : http://www.moneysavingexpert.com/phones/unlock-mobile-phone