Balancing the chemistry of the pool contents has turned out to be more of a problem than anticipated. Whether it was the quality of the water out of our well (we did not think so), the result of the huge rainfall since filling the pool or some other factor, the colour of the water remained distinctly green and slightly opaque. After repeated visit by the pool company, it was concluded that it was phosphates in the water (possibly due to the detergents used in cleaning up the inside of the pool before filling) reacting with the small amounts of chlorine. So a generous helping of a magic chemical was added. It had a small effect, but not enough to claim a fix. The pool company then had further consultations with their chemicals supplier, and the second conclusion was that it was naturally occurring trace amounts of iron or manganese. So more magic medicine was added, and at last things are going in the right direction.
After being advised that irrespective of the colour, the water was perfectly safe, we turned on the water heater on Thursday morning. After about five hours, the water temperature was an acceptable 79F, so was had our first ceremonial dip in the pool. It was very pleasant, and we stayed in for more than an hour (resulting in wrinkly skin on the fingers!). Plans are for a few more swimming days over the next week or so before we reluctantly declare the pool closed for the winter.