How to fill holes in internal blockwork

I took three times off my bathroom walls and under the tiles is a 5mm thick slab used to pack out the tiles. I've got this of but in quite big areas it has gone back another 5-10 sometimes 15-20mm to the blockwork. What can I use to fill in these holes? I'm planning to skim over this surface so does it need to be render, and can render be used this thick? Do I need to wet the holes to get the render to stick?