For the gaps in your knowledge, regarding your pet subject -

Slurry grouting - the engineer actually said no-one can predict what might happen if a slurry containing water was introduced into a 4000+ year old dry environment. One of the implications being that slurry might destabilise the structure.

Pressure grouting - rejected on the grounds that it was only normally done when you'd already created a sufficiently stable and strong structure.

With either method what you could not ensure was that all the voids would be filled.

Bye! Bye!

You can have your 'bat & ball' I'm off to enjoy the Vale of Pewsey (and Marden Henge), the Marlborough Downs (and Avebury) and Salisbury Plain (and Stonehenge) - its fabulous living around here. :-)