I took a few goes but finally I got what I was after.
Stray currents can cause holes in the radiator because when the electrons flow in an electrolyte (Water in this case) from + to - they take with them small amount of the metal. The same theory is used in electroplating.
I assumed that good earthing would overcome this problem but I was also seeing if anyone knew where they could be comming from. (headlights?)
I have a new radiator installed and I don't want to have a repeat performance of the last one.
Cheers
Mitch