My thinking was that specific roof extensions (e.g. dormer were covered by Class B), but that roofing for Class A does not..
Looking at the guidance notes (p5) the following text does seem to cover it:
When considering whether a development proposal is permitted development, all of the relevant Parts of the rules and all the Classes within those Parts need to be taken into account. So whilst Part 1 Class A prevents the installation, alteration or replacement of a chimney, flue or soil and vent pipe from being permitted development, Class G includes such development as permitted development subject to the rules set out under that Class. Similarly, changes to the roof of a house are not permitted development under Class A, but may be permitted development under Class B or C. For example, where a proposed two storey extension at the rear of a house has a roof that joins onto the main roof of the original house, the works will need to meet the requirements of both Class A (which covers the enlargement of the house) and Class C (which covers any alterations to the roof) in order to be permitted development. If the works also include the creation of a dormer window to enlarge the roof space, either in the extension or the original roof space, then they would also need to meet the requirements of Class B.
i.e. Following the red/blue example from the guidance notes: I think my roof plan above complies with both Class A and Class C (and as I am not adding a dormer, does not need to comply with Part B).