Geotextile, yes that was mentioned as having been used. That's probably the suspected "sleeve" I mentioned.

I agree about the chalk. It's a standard procedure to use a fill that's of similar porosity and density and thermal expansion characteristics as the surrounding rock, for obvious reasons. They don't use polystyrene, for the same obvious reasons...

In the end, they'll use this type of technique
http://www.ukqaa.org.uk/CaseStudyMonsHill/CS3-MonsHillSept2001.pdf
but with a chalk based grouting. It will be pumped in and will fill every nook and cranny and will be coloured so that posterity will know which it is. This solution, used on thousands of collapsing caverns in the West Midlands has been available all along.