Joining warm roof to pitched roof

[DIY_ME_UP]

Having a loft conversion done, we parted ways with our builder before Christmas due to poor work. 

New chippy has got us up to the stage we are now at, I am trying to finish off insulating myself.

 

Front roof is pitched, 140mm between and 40mm over with 50mm air gap. Runs up to the ceiling rafters.

 

The large box dormer roof is flat and runs full length with warm construction. 

 

I am struggling to figure out how I insulate the ceiling space and how to end it where the warm roof starts? 

 

Any help is appreciated. 

 

There is a small pitch which leads down onto the flat roof about 4 tiles worth.