Well... no steps forward, one step back.

Ok, so quickly, last thing I had her idling at approx 800rpm, with the man vac hose disconnected and blanked off. Thus proving that the leak is in one of the thin vac hoses.
Tonight, start to narrow down which vac hose it can be. I pulled the hose with the T-piece, blanked it off. Still hunted by about 500rpm. OK, so it's not any of them fed by that pipe. So disconnect the single one after that. Still hunting revs. OK so its neither

So after proving it definitely is one of them pipes the other night. I've now proven it's not!
So I thought, ok, lets just go through each of the 'bagpipes' and pull both hoses off, blank them off, and turn the engine over. Each time, still a hunting action.
OK, so I thought, let's just put things back to how they were the other night, back to a steadily idling car, and start again. Unplug the main vac hose from the plenum, blank off the plenum.... STILL hunting idle. AGGGH!!
I finally tried a little experiment, I unplugged and removed the ICV from its rubber mount, then used a spare ICV as a 'plug' for the hole. I then plugged the car's ICV back in, ran the engine, and spied down the hole. I was able to see a small amount of movement (only a few mm though) as I altered the revs and the car hunted. The car hunted in exactly the same way, which I imagine shows the ICV isn't at fault (as it wasn't actually helping the car control its idle)
Tonight's discovery are:
What I thought was wrong, isn't
The car starts to hunt at approx 95oC about the time the fans kick in.
The ICV doesn't appear to be at fault.
I have less hair on my head than I did at the start of the evening.

So where now, what other pipes are there to check?