DaggerDirk - that was what I thought as well, (I've long had a keen interest in Kapton matters) which was why I brought up the Saudia fire.
LMSC said no, it wasn't Kapton - it was something in the luggage. Pull the other one, I said - bearing in mind that was what SAA and Boeing said about the Helderberg which is now 90% certain to have been Kapton. Nope, really, was the response and they showed the detailed investigation report including photos.
In the case of wire arc tracking, the temperature is extremely high (10,000 deg C) that it has the same effect as a thermite lance. In other words, extreme concentrated energy.
In the case of the Saudia L1011, that wasn't the case - there was a fairly sizeable hole burnt in the cabin floor, and from the damage to bags, it certainly looked as if it had originated in the baggage rather than above, where the wiring bundles are located. Of course, the wiring (along with the floor or the aircraft and as you can see from the photo much of the upper fuselage) was melted in the fire.
As I said previously, the fire department's raking out of the baggage holds meant that precisely identifying the ignitor was not possible. The LMSC guy thought that it was quite probably something as basic as a box of matches - the old 'ignite-anywhere' Swan Vesta type.
I have, however, copied the Panorama transcript and forwarded it on the LMSC - we'll see what they have to say!