Thanks Chaps.
Interesting. When I had a look at the weekend I removed all the ground cables, and cleaned all the corrosion off with a combination of wd-40, wire brush and sand paper. Cleaned the point on the gearbox terminal, the chassis ground point, and both ends of the standard grounding cable back to bare metal. Finished off with some electrical contact cleaner and copper grease before refitting everything.
I also took the starter motor off aswell to clean all its terminals too as I've got a hard starting problem, and looking back now I noticed something...
I used to have a manual version of the same car before this, and the starter motor on that car had a ground cable attached to one of the big bolts which holds the starter to the gearbox.... the starter on this DSG version doesn't have that ground cable there though.
Im assuming the starter on the DSG is grounded through that grounding point on the DSG box?
Found a pic of the manual version here
http://www.myturbodiesel.com/1000q/a5/starter-removal-Jetta-mk5.htm
The connection at the blue arrow isnt present on my DSG. That connection on my DSG starts at the same place (ground point on the right of the yellow arrow) but connects to my gearbox instead of the starter. Is that how its meant to be on the DSGs??
If that connection is right, then I guess theres a problem with that standard grounding cable as it still didnt work even after I cleaned up all the terminals?
It is also possible for that grounding point in the engine bay to be damaged and not actually touching the grounded chassis, which seems unlikely?
If that speaker cable has taken over full responsibility of grounding the g'box and starter, then I guess you could be right in saying that there's higher resistance present messing things up as that speaker cable doesnt look as thick and is probably not as good quality as the standard item, plus about 4 times longer!
Will have to get a multimeter on it soon to figure out what values im getting where.