I have investigated this further and now think the problem is something to do with the sensitivity.  I am using 99% which I understand is very sensitive but normally get no Motion triggers.  However, this morning, a bright and sunny one, I got the camera to trigger with bright sunlight on my moving arm in front of  a dark background.  This was repeatable.  As was the failure (as before) to trigger with the sunlight blocked out and normal daylight on my moving arm.
I am in contact with D-Link Level 2 Support about this issue and have given them this information.