There's a number of options here, it might be that that the handset is already unlocked, being on tesco they actually use one of the main providers for their service so it might be worth trying another networks SIM in it to see. Also try the site below, it offers a number of websites that can give free unlocking (but for those reading this its not for all makes) if that doesn't work then for £10-20 you could get it unlocked at a local phone shop or market (try haggling this price down!) I hope one of these works for you.
ps. be wary if one you decide to go down the website route if they start asking for personal details, that usually means you'll end up paying money so you might as well use the market option
you can unlock your nokia 2760 using codes. you can get the codes either from your service provider or from other online providers like Mobileunlocksolutions.com