I took several XLR patch leads with shield disconnected. I used one of these at the destination end (room 2) which didnt make much difference. (It was quite difficult to judge as there were now lots of people setting up and making noise, very different therefore from the day before).
I tried a cable at control at the start of the 75m run. This seemed to lessen the main room FOH hum, but didnt cure it completely. I then noticed that the FOH hum was only coming out of one PA stack (both stacks were fed from same 16amp socket). I tried plugging stack into the AV crews gangs - still hum. Then I tried connecting the offending stack via loop out of the quiet stack - rather than xover - and humming disappeared.
Back to room 2: still hum, cant tell if better of worse cos too much ambient noise now, HPFs on boxes masked it adequately. Same in room three. Once FOH was solved in main room, room 2 and 3 were less crucial anyway.
So thats that. Im still not sure of exactly what was causing the prob. The non earthed IEC didnt seem to make any difference also on the second day. There was one test on first day where it didnt make difference also but i put this down to loose earth wire touching pin, i opened and taped it which, I'm sure, stopped the buzzing immediately. Too much going on too fast to be positive now tho, since it defo made no odds the next day.
I will always carry the right kit to diagnose and solve similiar probs in future. This was quite a big job, with lots of aspects that you seldom run into in the normal course of 'rock gig' saturday nights that I mostly do.
And thanks again too everyone.