"Janet Tweedy" wrote I keep the waterfall thing
going very gently which stops that end of the
pond totally freezing in winter

I do this too, but I have been told it's really a bad practice because you
are constantly drawing water up from the warmer pond and chilling it in the
air and on the rocks of the waterfall. Although it keeps part of the pond
clear and oxygenates the water, it also reduces the temperature of the whole
pond. (apparently when the pond freezes a gap is produced under the ice
which helps to "insulate" the pond)
But I still keep it running because of the aeration!