Just as a postscript re: the gear whine . . .
The 5th gear whine (at 110-120km/h) from last year was all but eliminated with the addition of a 125ml dose of Forté Diff & Gear treatment, but after the new driveshafts were fitted, the box was simply re-filled with fresh oil (the correct Total Gear-8, GL-4) but without the additive, as none was to hand.
The whine immediately returned, and the car has been driven for several days with the dull whine present - but have noticed that it is there not just at ∼110km/h in 5th, but also in 4th at around 60. So, it doesn't seem to be a solely 5th gear issue, and, even in 5th, there is no whine heard until speed reaches 110, when it disappears again at 120+.
Looks, then, like a resonance issue, which only occurs are specific speeds. Good news is that putting some Forté in the box again has killed the whine once more, so the stuff does seem to do what it says on the tin (or tube).
BX Tech talk