I have also been having this problem, so frustrating. It just loses mobile data connection and won't get it back. Also sometimes it loses normal connection too. It happens most when I get off the tube and enter the outside world again. Then I have to restart the phone which always works. Sometimes turning flight mode on/off does the trick, but sometimes even that doesn't work. Sometimes though I'll just be out in broad daylight and it still won't pick up any signal! I can't see it being a problem with T-Mobile, as my girlfriend could be sitting right next to me with her S3 on T-Mobile and she has no issues EVER.
Could it be that I am using a metal case? (probably cheap plastic covered in metal finish to be fair)
I really don't wanna have to send it in or whatever, that is hassle. It's just about tolerable at the moment, just.
So anyway james_uk I cannot answer your question, only add to it, sorry!