I have no idea. The property is currently being rented out so I'll need to wait until I can get back inside to estimate. My main two concerns about DIY-ing this are a) I dread to think what I will find underneath the current roofing so it might turn into a much larger job, b) I will need to get scaffolding because it is a second story flat.
Also, I have no idea who is responsible for the flat roof. There are 3 flats in the block and our deeds say that each flat is responsible for a third of the roofing costs. Does that still hold when a historical owner of my flat added these dormer extensions? Do I just suck it up because my flat is on the top floor and if I can get the roof re-roofed and better insulated I can enjoy the heat rising from my downstairs neighbours even better? I'm leaning towards the latter because I'm reasonably sure the ground floor will politely tell me to bog off.