[Q] no network topic

hi i got a p780 today i managed to get my rom backed up and rooted it and my imei number before i installed this rom http://forum.xda-developers.com/leno...-dual-t2795361. it went well but im in the uk and i have a virgin mobile contract . i had to order a new sim as my old one was to small . so i thought ill buy a payg sim on the same network . i cant get any signal at all i can see my number and it says its using t mobile but i cant get it working . i tried one of the new modems i seen at the top of this page but i flashed it and its still not working . any suggestions you guys are like experts . i have learnt so much already but i have no idea what to do now.