Thanks for all those who have commented. Since asking my original question, I have learned that whatever misgivings I have about my particular Tamron, it is probably no worse than others, or at least not fatally so. Which is probably just as well as an exchange is not on the cards. So, have I learned to stop worrying and love my Tammy? Not exactly.
I did go to a couple of local retailers to see how the CZ 16-80 felt, and to compare results with the Tamron. Compared to my previous test, these tests were far less controlled, but I'm grateful the sales reps let me out of their respective shops at all!
The CZ immediately looked snappier than the Tamron - something two of my kids happily confirmed - "that one dad!" Oddly though, at least to me, when I got to pixel peeping the results were far less clear cut, with the Tammy seeming to outperform the CZ in some circumstances on detail. However, the CZ just looked better which I suppose is the main thing. I assume this has to do with the coatings - and also I may well be at the extremes of what my 7D can deliver anyway.
You can probably tell where all this is heading - I suspect I will soon have a CZ on my wish list. I'm on the verge of an "out with the old and in with the new" stage of my life and as a kind of self-indulgent present to myself may well be seriously considering a CZ plus A700 package. If the Tax Office comes through with a reasonable refund it may well be a done deal.