Single-storey pitched roof insulation

[Keefa]

I have a single-storey pitched roof extension to my Victorian semi which houses the bathroom. There is no insulation in the roof at all and I would like to add some. There is currently a suspended ceiling and the roof is slate fixed straight to the rafters (i.e. no waterproof layer). I am planning to re-fit the bathroom, and possibly replace the ceiling but I don't know what the best way would be to insulate the room. It seems to be an issue of access if I use traditional wool insulation.
Anyone got any suggestions?

 

[masona]

There is no insulation in the roof at all and I would like to add some.

Where about in the roof there's no insulation, ceiling or roof rafters ? or both ?

 

[Keefa]

Both. It's just a ceiling suspended beneath an un-insulated, un-waterproofed pitched roof!