How should I plaster a hole in the wall on top of thermalite?
I'm from the US, so I'm not familiar with them either, but on page 42 and 45 of the Forterra product spec sheet it mentions plasterboard being adhered on dabs... which means using materials and methods like the ones found here: 

http://www.ultimatehandyman.co.uk/how-to/plastering/direct-bond