However I suppose its possible that some of the staff on there could not be working, and being transported to a site/depot to do work?
I honestly don't know. The rotation of power cars (2 on, 1 in for maintenance) would suggest that there would be no opportunity to do so.
43089, which was the battery power testbed, and IIRC Network Rail also used 43153 for a bit. Both now in service with First Great Western.Previous NMT power cars have been handed back into mainline passenger service (082? I believe was one).
I don't personally recall that it's ever happened.Has the reverse every happened with an NMT power car being loaned to a TOC temporarily?
Everyone on there will be crew, unless they are sanctioned visitors or technical staff. Sometimes crew travel "pass" if the train is transit (not undertaking test work, like ECS is to a passenger train). The facilities on board are normally used for presentation and "hospitality", using the train as a real time demonstration of what test trains do.
I'd add to jopsuk's wager; it'll more than likely be the last HST running in any sort of mainline service. It won't see passenger service again, more than likely.
The trailers have really only been modified in that the interiors have been completely stripped and refitted (except for Development Vehicle, that has a pantograph-a-motron). The power cars are standard, save for camera equipment, HS1 compliant cyclops headlight and a speedo marked additionally in kph. All electrical systems are the same, a GC power car has found it's way onto the NMT IIRC with no problems. Previous NMT power cars have been handed back into mainline passenger service (082? I believe was one).