Solved the issue! So I managed to load my original ROM file onto my external SD and install it from recovery. For anyone else who has this problem, I just loaded the zip file onto the SD, stuck it in my phone, powered off and clicked Volume Up Home and Power to get into recovery, (phone specific for Gs3 on getting to recovery) and went to install from zip and installed the file. Problem solved for me, although it did wipe all the data which isnt a big deal really. Phew. That was a scare. And it fixed my WiFi problem, so all in all a good learning experience for me
Anyhow, thanks for all the help, I appreciate it! And sorry for hitting up the wrong forum, its my first time on it.