After spending many hours on this I have had to give up.
I tried moving the anchor to various places. What would work with one file would not work with another. Sometimes the point jumped to was too early, sometimes too late.
I tried dividing up the files so that one xhtml file had its start point where I wanted the start position to be. This did not help at all. It would still jump to a position in the previous file!
So where is the bug? It might be in Kindlegen. If not it must be in the Kindle devices. I hope that Amazon will sort this out soon.
Until Amazon fixes this I don't believe it is possible to find a reliable solution which works with all devices.