Flat roof utility room - order of refurbishment?

[stoobloo]

We have a utility room with a flat roof, and it's clearly had a leak in the past as there's a bulge in part of the plaster ceiling.

As we will replace the felt roof sometime, I wondered what order we should deal with the plastboard roof and any insulation (currently it's uninsulated).

Would it make sense to deal with the felt roof first, or the internal first? Would the insulation form part of the internal roof or the external?

 

[Alastairreid]

Warm roof external, insulation fixed above decking.
Plasterboard etc once roof is complete.