Well, I finally got this all resolved so I thought I'd come back and report. First off for Hackney plumbing: I never did try exactly what you had mentioned so I can't say what the result of that was. To ballvalve: regarding your comment: "When you open a hot tap, the cold water cannot flow in the opposite direction. I dont care where your bag in a can is located." You're right, and this was my "fix" for getting cold water more quickly - turn on a hot water tap at another location, then turn on the cold where I was getting a drink. Since the hot water can't flow both ways that seemed to do the trick.
But more to the point, this morning the original plumber finally came back and took off the expansion tank from the hot side and put a new smaller one on the cold side. This seems to have fixed it as was expected. Yea!! He said he had never seen that happen, but was nice enough about it. I just wish he wouldn't have been so hard to track down and get out to fix it. But, all's well that ends well I suppose.