My first encounter with Thermalite was in the '70s. Hole, plug, screw, crack
running both up and down, through the mortar and the adjacent blocks. Hated
the stuff ever since.

My first encounter with them was also in the seventies the guy we bought our first house from and a neighbour used some lightweight concrete blocks bizzarly a mixture of wood chips and concrete to build garden walls then wondered why the blocks disintegrated, the neighbour told me they were Thermalite.

The second house had a garage and kitchen extension built and the internal walls were the traditional grey Thermalite stamped version (spit). The only anchors that worked effectively and crucially allowed you undo the screw without loosening the fixing were made by Fischer. They were a nylon plug with four helical vanes down the side in a slow spiral that had to be hammered into a pilot hole, could sometimes be a problem near a mortar line but generally an excellent fixing. I note they are no longer made but instead Fisher recommend their universal plugs for aerated concrete, don't know how effective they are not having any Thermalite in our current house.

I also experienced your cracking problem when two Mr. Men installed a roller door at the previous garage, I told them not use hammer action on their drills, no they knew better, twice as many holes later they finally managed to get some sound fixings in place.

Richard