I have a feeling it's just not used. They also have to upgrade quite a few PE passengers, as they're missing 20 or so PE seats from upstairs when this happens.
Thought that would be the case. Thinking about it, the same staff would be on the aircraft whatever the specification. Therefore, no IFBT normally, means no IFBT if there is space for one!
Should have thought that one through a bit more really! [:w]