Just to update this discussion, I bought - more for a bit of fun than any
other reason - an ultra-cheap solar powered pond pump, to see if I could get
the measure of the concept. It turned out to be one of those things which
when it works works perfectly well but which very often doesn't work at all.

This isn't just a matter of sun or shade: the pump doesn't seem able to get
itself going again after even the briefest spell of non-sunlight. Only
completely obscuring the solar panel for a few minutes and then uncovering
it again can persuade the water to flow.