Replying with thoughts only and no experience of such a problem,so forgive any drivel
I am sure you have considered this but here goes anyway
Maybe a backwards procedure instead of looking at the overall picture
You cant measure the leakage at the main panel because the armourings are all glanded in common
You cant insulation resistance test because of isolation problems
You cant isolate willy nilly because of safety and operating issues
Did the problem occur only after installation of the supply transformers
If you assume that the swa s have not deteriorated and insulation values may be sound
It may be long drawn out,but may get you where you want
Working on the extremities of the installation at a final circuit level with your earth leakage clampmeter may well be one way of highlighting the issue
Beginning at the distribution boards and getting a total for each board
Highlighting those with high earth leakage and then pinpointing to which circuits the leakage belongs, and then looking at whats used on those circuits, any unintended leakage may be highlighted
Perhaps the leakage is simply a factor of the equipment used and that the existing installation was not designed with earth leakage as a functional property at that time
I just read that leakage is not permitted to exceed more than 1 ten thousandth of maximum demand
Does yours