After writing the post on the wall construction, I double checked the costings for it. The use of Larsen trusses was to avoid the cost of having a complete web of OSB, the extra separation between columns of insulation being provided by geotextile (a thin synthetic scrim) stapled to the uprights. However it seems that geotextile is actually more expensive than the OSB web so our trusses will now have a continuous web from floor to roof. This is cheaper and easier to manufacturer.