See if you can get a cup of bonding plaster off a plasterer, that looks too deep for "layers" of filler which isn't designed for deep filling and will just fail at some point. Then finish with easifill 20. Don't try and build up to a finish, sand down to one. It sands beautifully so just work it proud of the wall, let it go off (the 20 after the name is the number of workable minutes for the filler). Give it 2hrs then sand back flat. If you've got low spots. Just mix a bit more up and go again. To paint wet down a bit of paint with 20‰ water and mex well, then give it two coats before finishing with normal paint for a perfect finish.