I'm going back to my original design
Frame built out of scaffolding poles with KeeKlamp joints. Already have cut poles to sizes and have masses of KeeKlamps so makes sense to use them. Frame will be on "stilts" rather than on the ground due to sloping land. That way when land terraced the shed won't be on infill and there no chance of settlement. Unistrut for floor bearers every 12". In between frame poles will be 3x2 timber.
That will give me the basic structure. Door and window will be used UPVC (eBay ?). Outer cladding still to be decided but am having a "chuff it" moment and might even opt for box profile steel and ignore how it looks to other people !
Great shed, id change the polythene for breathable roofing membrane,and a lay of damp coarse between the wood and brick base to stop damp on the base wood