On a visit to Millbank, I got reprimanded for using someones first name, someone I had worked with previously. Yes they were way up in the pecking order, but this irritating little man insisted that my lowly status meant that I had to use MR xxxxx.
I then got into further trouble for telling him where to stick it. In the end he backed down, probably as he became very unsure as to how I knew such high-ranking people.