Iphone help

[super__dan]

I am proper stuck.

I have a 3G that was on 3.1.2 that I unlocked myself and been using happily on a tesco sim card (having previously used on O2).

Whilst tipsy on friday I decided to finally accept ITunes offer to 'upgrade' software to 4.2.1 (latest one for 3G), which has screwed me right over.

I'ts now on 4.2.1 (8C148) and baseband 05.15.04 (which having read up since seems to be my problem), I've JB'd it and ultrasn0w'd it with no joy I get no service no matter what I do. The phone works perfectly on wifi as a touch but need it as a phone. I've even bought a O2 PAYG sim to try but this doesn't work either so it's defo the phone that's crocked.

Any advice very much appreciated!