I have a hole in my wall which is about 300x300mm, approx. 30mm deep going back to thermalite block. I only have experience with smaller patching-up jobs, so I'm unsure how to approach it.
My instinct is to use a piece of plasterboard, somehow attaching it to the thermalite, filling in the gap with plaster mix, then using plaster tape and smoothing with another layer of plaster mix. Is this the right approach?
If this is right, what should I use to adhere the plaster board to the thermalite?