You can easily test the makeup of the block work behind with a drill. If it's thermalite you will find that you can drill it with a regular drill bit and no hammer. This is not to say you want to do your final fixing this way, it's just to establish what's back there. When fixing to thermalite type blocks, use a masonry bit with no hammer. Thermalite just turns to mush using a hammer action. Without the hammer you will end up with a true hole and not a crumbly mess. I have used rigifix fixings on our dot and dab/thermalite walls and you can hang off them once installed.