Hi My builders have just started to build my external walls and have used a thermalite block to construct the external skin on my extension on one side , this side is next to my neighbours extension and will not be visible.
Is this construction okay. They have used brick for the internal wall.

I've seen thermolite used for both internal and external skin, but never with brick on the internal.  It can be used for external skins, but is very rarely done, and only where weight of above can accomodate it.

Thermolite blocks are like blotting paper if external without any waterproof coating.