The problem is listed as above - and was my fault.
Basically, i've done quite a few of these PS2s recently, and I lost concentration for a brief second and accidently soldered the G wire to the B pad on the mod and vise-versa.
So there's a lesson, to check the VIA connections correspond to the correct points on the BGA before reassembly. It could really save you time and confusion in the long run (that and don't try and install mods at 12 o'clock midnight - or else you'll find yourself doing something as trivial as this...
