+1 on adding polystyrene (sp) panels to he underside of the corrugated sheet though as t.o.p was concerned about the condensation rotting the joists/purlins perhaps insulation in the form of expanding foam injected between the sheets and the timber would solve that problem.
Personally I would not be concerned about the timber being rotted as I would imagine it would dry out once the condensation had gone and it will probably outlast the asbestos roof anyway