An ex library book about ponds (purchased at St Neots library a fortnight ago) recommended another way to get rid of surface pond algae.  Last year we tipped a full bale of barley straw into the pond.  There it floats still but with no obvious effect on the algae it was supposed to clear.  A neighbour gave us two sturdy nets that she had used  for her horses and we will try again  – as suggested by the author of the book.