"And a question to the guys in the know: how is a RAT probe heat fault to be 'isolated'? is there a switch on the IP? or can it only be done by pulling the relevant CB, with the known consequences..."
Need to see more circuit diagram..........Item like that would normally have it's own protection. If it went short circuit you wouldn't want it taking out all the probe heating when the fuse/cb popped.
But as far as this is concerned the probe heat was working it seems.