Didn't get a chance to get on here until now but I fixed it last night.
I did this by going back into reboot recovery mode (by pressing HOME and POWER), going to partition settings, COMPLETELY wiping everything on the sd card, mounting the USB and dragging a new ROM .zip file onto it. (I tired this twice without completely wiping and the phone continued to have problems booting.
The flash worked fine that time and all has been well. Turns out the first one I was trying to apply to it was just an upgrade.
Thanks everyone for your help, makes perfect sense now.