I have a few ideas to mind:
Have you tried to enter the APN settings again?
Try and connect to wifi and download the giffgaff apn app from Play Store.
You can then set up the APN by following the instructions and copying and pasting the info into the relevant fields.
Where was the phone purchased from?
Another issue could be that maybe the IMEI might be blocked due to the original owner reporting it lost or stolen?
Have you tried another sim in the phone to test if calls, texts and data work ok?
I hope this helps mate.
Just put sim in old phone and got connection, loads of texts coming through so I know the SIM is okay, where would I find the IMEI number on my phone to check it? Think this may be paprt of the issue as I've tried the APN again and manual roaming a few times and phone has been on and off loads
Since 4pm yesterday, as stated above I've just put sim in old phone and the SIM IS FINE. I've got the IEMI number but how do I find out if my phone's been reported as lost? Or are there any settings other than manual roaming and APN to get it working?