[Q] Not registered on Network topic

Hi, brought an old Galaxy S3 i9300, on JB 4.1.2, from ebay, it was locked to 3 mobile (uk) and worked ok. I bought the phone for my other Orange/EE account and so got the phone unlocked. It worked fine on Orange after the unlocking. After a week I was offered an OTA software update which I accepted, I expected it to be to JB 4.3. After completing the update the phone rebooted but never got passed the second Samsung logo, it hung there. I carried out a factory reset and cleared the cache. This did not change anything it still hung on the second Samsung logo. So I then reflashed using Odin with 4.1.2 i9300XXELLLA. The phone booted and the software was installed but then after trying to make a call I got "not registered on network", even though I had scanned the networks and both manually selected the correct network and allowed it to 'select automatically'. I removed the battery, carried out a reset and cleaned the cache, no inprovement. I located another download site of the i9300XXELLA and reflashed this using Odin. Still the same 'not registered on network'.
Does anyone have any ideas of what went wrong and how to correct this?