I don't think you'll be finding hydrazine on a pax-carrying aircraft anytime soon! And it wasn't an APU as such, but a source of power for non-normals potentially found in the flight test programme. M2D and ChristiaanJwill know far more.
The charter flights were different insofar as they often went to non-BA stations, so there was a bit more donkeywork to be done to get all the paperwork organised, but nothing a regular charter pilot won't be used to. Generally one would get a fuel plan/flight plan filed from Ops at LHR, but apart from LHR/JFK/BGI/IAD (and presumably MIA/BAH/SIN in earlier days) we produced our own loadsheet. I only once had to produce a fuel plan/route plan from scratch and that was at Sondrestrom (as it was) with a dodgy fax line. You'd have to file a flight plan occasionally.
We carried a '
PR' on most of them - a line pilot or FE - to carry out a running PA and do general liaison. They were volunteered to do the loadsheet.
The atmosphere on board was very different - these were pleasure flights and so were the opposite of the JFK business run. Landlady may be able to elaborate on this.
The round-the-worlds were just a big charter in this respect. As you note we carried a 'flying spanner', since Concorde-qualified LAEs are hard to come by downroute. It looked like a great job on paper, but they were often at the airport for many hours before or after the sectors carrying out routine maintenance or dealing with snags.
I enjoyed the charters a lot - everyone was geared up for a good time and in general the flight had something different for us, too: Whether a lightweight departure on a 'round-the-bay', squeezing into a short runway (e.g. Bournemouth), visiting SFJ or Rovaniemi, or setting off around the world, predominantly to non-BA destinations. My favourites, though, were the RTBs out of Filton - EVERYONE was either connected to Conc development or manufacture, or was related to someone who was. Fantastic atmosphere. Shame the runway wasn't a bit longer........