Not sure I follow. The cases I labelled a and b appear to be specifically permitted by the ANO [130(9)].
bookworm: I think (a) is OK but (b) in your post is not. The reason is that the client paying either you direct or your employer indirectly is valuable consideration for the flight. The position might be different if the client was in the aircraft and paying his share of the direct costs of the flight. Paying you to visit him or similar is quite different.