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I want to add a PIR sensor to a corner light. It's on the far end of the house so anyone approaching will be coming from the right.
At the moment my thoughts are to disconnect the cable at the light fitting, running it as far to the right of the downpipe as the length will allow, then run back to the light. However, I'm wondering if the heat from the boiler flu is likely to trigger the light. My gut feeling is it won't since the residual hot gas will have dissipated, but I'd like your thoughts based on experience.

If you are asking will the boiler flue gasses activate a PIR that operates a light the answer is probably.
Without going into the technicalities of how PIRs work you would be better off if you could mount the PIR lower than the flue.

As an aside, I never have liked floodlight/PIR combos, I recommend getting separates, that way you can put a light where you like and the PIR "looking" at where it is required.

If I mounted the PIR lower than the flue, folk would bash into it as they walked past.

I could joint the cable and run it further along the wall so it's well clear of the flu I suppose. Small jobs always turn out to take the longest.