Well I know it was on 02 as the girl I brought it off had that network on it am running giffgaff as we speak and I have also used EE SIM on the phone and it worked fine the only issue I had was with the virgin sim that I have not used yet and had it since last Monday.
I've worked it out now. You have to turn on find my iPhone. And login with your Apple ID and password. It will then let the sim work. But this then has to be done each time you swap it back to another network.