I do have a breathable membrane on the roofing OSB, followed by the felt and then the sheeting, so hopefully that should give me enough air-flow to avoid condensation, but also to provide a nice warm roof. It will be insulated from the inside too.
Wizer, I still have quite a lot of the breathable membrane left if you want it for your workshop, should be enough for your roof and your walls.
I would really recommend the Onduline Sheeting for your roof - really fast to install, easy, light and not to costly.
Although, if I were you, I would get a good plastic adhesive to stick the screw caps down, as with others and myself, they do pop up, or sometimes don't even clip down properly, although most are ok. Again, I have a fair few left over if you wish to use them - I have no other use for them now.