As noted above, I follow the travel policy of my clients. My current client allows C over 5 hours but requires that all tickets are fully flexible. As my work has very clear time limitations I tend to book less flexible fares, which means my travel costs are around 40% lower than the client's employees. They've even mentioned it when I've submitted the fee note, but I just brush ot off as having spotted a bargain fare!

Now I'm not that naive that I don't understand about airlines' retrospective discounts.....