Many thanks for all this good advice. I typically use this lens on holiday, often when walking around a European city with my wife and non-photographer friends, who don't wait while I am taking photos. So a dual lens solution would probably lead to someone getting lost!
And yes, the loss of 18 to 24 would be significant. The 16-85 is obviously a good lens of its kind, provides a little bit of extra reach at either end compared with my 18-70 and is presumably optically better, and has VR. Sounds convincing to me, particularly as a used buy.