Just did some reading and the capillary sensor is probably a fire valve, sounds like they trigger at 66 deg C. Looks like most are connected to the oil feed, this one goes to an electrical box, maybe it controls power to the oil pump. I can't see the aga being on fire and any of the circuits being fully loaded at the same time, or not for long at least. Plus the higher temperature would be at the top, away from the cables.

Not sure about the AGA but in general , if you can keep your wiring below the hot part - the firebox of a solid fuel thing, and the burner tray of a gas or oil burner, the convection that the heater causes works for you rather than against you as at the bottom it is pulling cold air in 

Being above, in the rising hot air, is to be avoided. Also, whlle you have the drill - where is the ventilation - it is good to be in astream of incoming cool air if possible.

There is still radiant heat to consider, but any obstacle, but ideally a white painted or metal shiny one,  that breaks the line of sight from the heat source to the cable, will solve that.  A simple L shaped upstand of sheet metal could achieve that - if radiant heat is even a problem ,

If it is looking marginal, then a test is possible, then to put a couple of the temperature revealing stickers on something like about the size of a jam jar iid one black and one shny, and push the to the back to the proposed cable location, and leave them there for a week, and see what the peak temperature was...

Mike.

Might want to think about protection from rodent damage too if very low down. Sounds an ideal home for them.

Asking the silly questions first... as the room presumably has three other walls, could the problem be designed out by running the cables the other way around the room, or even down from the ceiling? (presumably the Aga doesn't join the "island" to the wall? - I probably not picturing it adequately)

    - Andy.

Alan B said:

I am thinking of running all the circuits in 90 C thermosetting

But the operating temperature of the conductor should still be limited to 70 deg C unless you have accessories, JB's or terminals that are suitable for the higher temperature?

Put a bar of chocolate down there for a week. That will tell you if it gets toasty or if you have vermin.    ( I doubt it would stay there long in my house mind )  

Gary

If the choc doesn't melt then the temp is probably below 30C.  Do you actually mean behind the Aga?  Is there a gap between the Aga and the wall.  Normally they are fitted hard up to the wall.

Nice one!

Isn't the whole concept of Aga cookers that they reduce fuel consumption (compared with a traditional range) by virtue of being well-insulated?

So long as the ends of the conductors are well clear of the Aga, I don't suppose they will be anywhere near 70C.

Another thought, "drilling through the side of the fireplace", there might be a void in there, or worse it is a working flue for a fireplace below. Chimneys are notorious for bad/loose brickwork because they were never seen and solid fuel gases attack the mortar. So you might want to think about putting a tube through and putting the cables in the tube, to protect them from falling debris within any void.