I did some testing using lat/lon values to retrieve hourly10day forecasts which all returned a sunset/sunrise value.
If I change this to use a city name, I tried Liverpool, the sun_Phase values are no longer returned. Is this by design or is this a bug?
Ah ha... I think I have worked it out.
Just using Liverpool is not enough to identify a single location as there are several worldwide so the sun_Phase cannot be determined. Changing it to 'Liverpool,UK' works just fine.