OK, just an update for the OP.
I tried the adb route via PC using http://forum.xda-developers.com/show....php?t=2393289
. Failed four times. Updated and re-downloaded adb and drivers three times! Everything looked like it should, with 2 missing file errors, but phone status never altered. I have no idea what I was doing wrong there, but obviously was. I thought that one of those files may have some importance after all, the other I am sure was useless though.
I tried the EFS Pro, but that wouldn't complete. I tried SGSIII EFS Backup/Restore, and it seemed that it would only restore its own img backups, not my lowly tar.gz file. I tried kTool and that wouldn't even find my file, let alone try to restore it. I assume that also works with img format? I was beginning to think that my backup may have been corrupt as nothing seemed to want to use it.
I tried flashing my tar.gz backup via Odin, but that just crashed Odin. So I extracted my file to a tar (within the gz file) and that crashed odin too. Probably just as well as it would probably have bricked my phone - but I was prepared to try anything at this point.
So, I decided to root my phone again and manually copy everything over from the extracted efs file and overwrote the efs on my root folder. But instead of using ADB, I opted for a different way. I then downloaded and installed busybox and terminal for android and typed out:
chmod 664 /efs/00000000.authtokcont
chmod 755 /efs/FactoryApp
chmod 775 /efs/bluetooth
chmod 775 /efs/drm
chmod 666 /efs/gyro_cal_data
chmod 644 /efs/h2k.dat
chmod 775 /efs/imei
chmod 700 /efs/lost+found
chmod 600 /efs/nv.log
chmod 700 /efs/nv_data.bin
chmod 700 /efs/nv_data.bin.md5
chmod 775 /efs/wifi
chmod 644 /efs/wv.keys
just as I had done whilst on the computer via adb shell. I got two errors this time, lost and found folder didn't exist and 00000000.authtokcont didn't exist. I then typed reboot, it did. I then went to make a drink when I heard my phone vibrating itself silly. I went back to see that I had recieved three days worth of texts and missed call alerts. Boy, am I happy! And to think, when I rang O2 the other day and spoke to one of their "gurus", their diagnosis was that my sim card may have failed so would send me out a replacement!
Thank you XDA