I like the idea that the induction loop is being picked up by the guitar and the amp; the amp has a spring reverb. That explains the wierd signal pickup. I check that tonight.

So now we are left with the strange wiring behaviour, and I will now be checking every XLR and jack in the place!

The intermediate box looks like a normal unit with 12 xlrs feeding the desk and 4 "outputs" coming from the desk as (spooky music here) Jack sockets. The stage box is at the "other end" of a load of XLR plugs which are plugged into this intermediate box. For reasons unknown the stage box is a "16 in 4 out" all XLR unit with a big label next to inputs 13-16 of "Not in use". This leads me to wonder whether the stage boxes were purchased separately from the multicores and have therefore all been wired on-site.

The system was installed by a contractor who may be contactable but I don't know if the same guy was dragged in when "the loop went funny" some months back. By total coincidence, it turns out that I work at the same place as the bloke is the sound engineer for another local church, and whilst I don't want to lumber him with this setup on a permanent basis I'm going to seek his advice as well

Once again, thanks for your help. I think we are getting somewhere!