The substrate is plasterboard, not skimmed. I am doing two rooms, in one the tiles are 450 * 300 and the others are 600 * 300. The pack of the first tile weighs 28 kgs and holds 12 tiles, so they should be 2.33 kg each. The larger tiles are 26kgs for 8 tiles, 3.25 kg each.
The tiles came from different suppliers and both are adamant that the tub is OK. As you say, its bad enough fixing them, so we certainly want them to stay on the wall.