Put it in your Freezer for about 3 hours and leave the Simm card in it, and keep the phone switched on.
If you hear some rattling and banging in the Freezer, Do not worry?
I will only be the Chicken drumsticks & Fish Fingers Dancing to you ringtone tune, because someone is trying to call you! hehe!!!
Take it to any independent phone shop, they will unlock it for you for around £10, they have all the data cables for all Tesco phons now, because everyone is unlocking after the free voucher runs out, and want to put their original T-Mobile 02 Voda etc in them.