Tesco mobile

b56c

My daughter saved up her money and bought a sony ericsson from tesco, getting £20 off when you spent x amount. She switched on at home and everything was fine. However, as she intended to switch over to t-mobile, I inserted my sim card to see if the mobile was locked. After doing this, a message was displayed asking me to insert a sim card, so I took that to mean that it was locked. So far so good. However, once we returned the original tesco sim to the phone, the same message continued to appear, making the phone completely useless, and so I returned the phone to the shop for a replacement, without going into all of the details. This experience has made me wary of tampering with her phone and wonder whether this was just a fluke problem and whether we might experience problems if we pay to have the phone unlocked. If unlocking/putting in a sim from another network does render the phone unuseable, what rights does the consumer have re the original seller?