As far as I'm concerned the foldback signal is coming out of the desk back to the stage, therefore so to me that makes it an output from the stage box.

Anyhow, had another fiddle today and suspicions point at the Induction Loop system, or the wiring in the vicinity. If the Loop amp is turned off we don't get feedback.

The induction loop amplifier is in the same area as the main PA amps. Found out some more about the wiring. Theres a box by this gear which is the one hooked up to the desk; the stagebox that is by the stage is connected into this intermediate box by a load of XLRs.

Still think it's a crossed wire or two somewhere. At this intermediate box, if I unplug the balanced jacks that go to the amps for the main speaker system all goes quiet, if I touch the jacks to any exposed metal in the stagebox that is connected to the mixer the signal reappears!

What fun. I'll be noseying further on Thursday.