What I'd not done was to properly try out the lens after the drop. It appeared fine when I was giving it an initial check, so I assumed all was OK. But on Sunday we had family down from York, and it being a nice day we took them out on the Heath.
That is where I discovered that the 18-200 VR lens was refusing to focus properly at infinity when zoomed all the way open. It was fine at shorter ranges, and fine focusing to infinity when at the 18mm end of the range. But attempting to use the zoom to focus on something a good distance away?
Nope. Just a blur, with auto or manual.
This is the most expensive bit of photographic kit I've ever bought, and not being able to use it for longer ranges rather defeats the purpose.
Getting one of these repaired looks like a three figure sum — not as much as it cost, not as much as it would cost to replace, but still a fair sum. But then I discovered something on the 'net. This symptom can be caused by a loose front element. There's a ring that holds the front-most bit of glass in, and if that's loose and the glass has ridden forwards, it can't focus. But if you tighten the ring back in, it may work.
And yes, the front element ring was loose, and yes, I could turn it with my fingernails, and it took about a complete turn.
So I now have a properly-working-at-full-zoom-and-infini