MMS not working Lumia 900

[WPCentral Question]

So i recently sent my phone off to get the screen repaired. When i sent it off i completely forgot to take the sim out so 2 new things 1. new sim but same number and 2. my phone was reset. since i've had it back i can send messages, makes calls, connect to internet with or without wifi fine but i can't send picture messages. I've tried the whole adding an APN thing and its not working. I'm not on tmobile, im on Tesco.