I talked to the designers again today, they'd designed the shed as if it had a concrete roof (because I told them I'd be hanging timber etc. off the rafters) So the rafters were 225x50 at 600mm centres, the plates and studs were 100x50 at 450mm centres, plus a ribbon plate (100x38)

The rafters will now be 175x50 at 900 centres. The plates and studs, 75x50 at 450mm centres and no ribbon plate.

Sizes;
Bearers 200x75/2.4m at 2.2m centres
Joists 150x50/2.2m at 450mm centres
Plates 75x50
Studs 75x50/3.0m at 450mm centres
Rafters 175x50/4.0m at 900mm centres
Collar Ties 75x50
Ridge Board 225x25