I tried chemicals, but they were expensive, short-term and made the lillies
go spotty. Then I tried Fairy Moss to shade the pond and use up some of the
nutrients. It grows at an astonishing rate but it's not too much of a pain
to control by scooping off once a week. It has really clobbered the blanket
weed!
A side benefit - the local heron is looking a lot leaner now the pond
residents have extra shelter, and the compost heap is getting regular
supplements.

Scooping out the blanket weed was always a worrying time - it used to snare
newts and dragonfly larvae, so I had to gently check every last strand to
make sure they were all released back to the pond. The fairy moss is much
better because it has short dangly roots which don't trap wildlife.