Extension inner wall.

[frales]

Hi

I have been left high and dry by some builders and I am now trying to finish an extension. I have checked cavity wall projects for the answer but it is not clear.

As far as I can see the walls are of breeze block on the outside, a cavity and then timber frame. Then there is wool type insulation, a sheet of polythene and then plasterboard. Is this correct?

Should internal walls be the same or only walls to the outside of extension.

Any advice asap greatly appreciated.