Maybe the blocks are Thermalite which is a very soft breeze block (can be cut with a wood saw). Possibly the hose is a framed construction with only plaserboard, frame and insulation between inside and the external brickwork.
Maybe it's not surprising the party walls are made of a different material. It is probably load bearing, and needs a denser construction to be soundproof. Stronger as well to prevent active DIY neigbours from knocking holes in each other's walls.
Thermalite is a light powdery dust made from power station ash. Can be scratched with your nail, and has a strength and weight one up from polystyrene.
Rawlplugs will hold to a point, but as the blocks are so soft, will pull out if under excessive loads.
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