Right so I got around to “fixing” my phone. It had been freezing and restarting itself constantly so after a bit of research the obvious solution seemed to be to flash it with a more recent firmware and it would obviously work perfectly from there. Well, the result was that though the flash worked perfectly and everything seemed super fine I ended up with a warped IMEI number. This basically means that the phone can no longer connect to any network because it's got a bollocks identification number. Changing IMEI is a complete pain because it's technically an illegal act. I'd need some kind of custom hardware to do it, it seems.
Worst thing was that there is a method of setting the IMEI but I only discovered half way through it only works for the US version of the S3 as the International version doesn't seem to support RMNET + DM + MODEM USB mode. Agony.
All in all, I might as well have dumped my phone in the liffey. It's pretty much borked now. Still, the positives are that I learned a lot about how android phones work and I had yet another lesson in backing up everything. (Not just with zip tools either, because they often lie.) A tip for everybody out there, do not do anything with your EFS folder lightly, it's not restored when flashing.
Anyway, I can pick up a cheap samsung phone from argos until such time comes that I can either buy a new smart phone or have the IMEI corrected. Pity, because otherwise my flashing it seems to have corrected all the problems I was experiencing. Biggest thing I'll miss is definitely the tethering mode which was really handy when working in coffee shops that didn't have wifi.
Pity I screwed the pooch but again it's yet another lesson that you shouldn't rush into things without due diligence.