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.