Thanks Andrew,
The 8300 chip is significantly smaller, so it should be better than the FF chip. I'll have to do some test shots on my D800 and see how it compares for myself - might be whinging about nothing!
Link - this is a shot taken with an 85mm prime lens mounted on the CCD (before I'd made the bracket though). The stars are much tighter in the corners.
I was presuming that the focusing mechanism would move lenses inside relative to one another. Given that I was focusing on an object at infinity when the focus distance scale was reading 0.3m would mean that the optics wouldn't be where they should be to produce sharp focus across the field of view. I guess the prime lenses might suffer the same fate, but I hadn't seen it with the 85mm lens. I wonder if the 14-24mm lens is such an extreme case that it is more sensitive to a slight change in backfocus. I had the lens stopped down to ~f4 - an imprecise science given the lack of an aperture ring.