I've got a shed going up next week, and I intend to insulate most of
the inside (a single skin partition will seperate out the other
section)

Sounds very nice. Only things I would suggest a

1. Use 50mm of celotex. For a shed, 75mm would be overkill. This enables you
to leave a 25mm ventilation gap between the outside skin and the
plasterboard. 75mm would have the slight advantage of allowing cross layed
boards to reduce cold bridging through the studs (and possible condensation
effects), but for a shed, I doubt it is really necessary. It will be as warm
and comfortable as a house which ever you do.

2. Don't bother with the plaster skim. Just tape and fill taper edge boards.

3. Insulate the whole lot and then separate out the partition. Insulate the
partition wall if you want a temperature difference across the rooms. This
gives you the flexibility to remove the partition in the future to get a
bigger room. It also helps to ensure that the uninsulated end doesn't rot
before the rest of the building.

Christian.