What to do with this wall/ceiling?

Chet T16

I'm doing some work in the house, blocking up some windows, replacing a door with a window and insulated plasterboard on the external walls.

The 2 rooms I'm doing are in the "extension" which is a garage that was converted and attached to the house around 1994, the house was built in 1992. Both garage and house are 9 inch hollow blocks with battened plasterboard. The house has fibreglass behind the plasterboard but the extension doesn't. The ceilings in the extension are 2.75m which I always assumed was because it was a garage. Today I started taking down the plasterboard and found something unexpected!

I'm completely confused about what has gone on here, or how they have done it. Is the roof still original height where the single block wall was in the attic space? The original ceiling would've been 2.25m

What should I do with the top bit of the wall as there would be quite a void behind if I put the plasterboard up

Chet T16

Answering at least some of my questions here. After a bit of googling I came across the term "stub truss" and it all makes sense. There must've been the low ceiling in the garage with decent attic space. To raise the ceiling the only option was to move joists up to the next wall where they now sit on another wall plate. These joists are obviously beside the truss and I assume (hope!) properly tied into it.

I dug out the survey from when we bought 9 years ago and it only mentions a binder installed across the joists.

I plan on putting some 100mm PIR in the void, filling the space between the solid wall and the battens so keeping the 1 inch gap behind the insulated boards. I'm open to suggestions on that!