Thanks for everyone's input.
Following a few test drives and phone calls over the 'twixtmas' period, I have put a deposit on a 6 month old Corolla 2.0 Excel Sports Touring with the panoramic roof (an opening sunroof is feature I have had on most of my cars over the years and didn't want to forego but which is all but impossible to find on an NX).
As many of you said, the UX is small. The driving position felt quite cramped and the boot is even smaller than I had imagined. The NX was nice, although I didn't drive one, but it felt as if I might as well stick with my RX if I were going to another SUV of that size.
Sure the Corolla is a step down from the RX, but it's going to give me an extra 20ish mpg. Meanwhile, my Lexus SC430 is asleep for the winter and keeps me with the Lexus brand for summer motoring.