For the avoidance of doubt, HA has never criticised the polystyrene, though people who sign the petition such as wintersylph are naturally shocked to learn that that's all that's been done in 4 years and say so. Whether it was the right quick temporary measure isn't something we'd know, since as you say the expert authority for using it hasn't been published so we all have to take it on trust that it was.

The trouble is – it wasn't quick, since it wasn't provided for many months – until the initial measure, an entirely inadequate rain cover, had buckled and let in the rain and precipitated a further huge collapse. In addition, it's turned out to be semi-permanent and therefore an unprecedented measure. So we're justified in worrying, since the data doesn't exist, that it may not now be preventing further problems developing. Any volunteers to be the one to remove it?

So our only thoughts about the polystyrene is that it was applied too late and too long. Three cheers for the polystyrene, it's reversible. But none for the damage, which isn't.