After reading several websites that suggested covering a pond and
keeping the filter running during the winter I decided to give it a go.
My first attempt has used bigbubble bubble wrap covering about 70% of
the pond area and insulating the filter. This has been a spectacular
success with the water temperature increasing from 7 to 10 Celsius over
1 week. However its a bit unsightly, doesn't look if it'll have much
longeviety and its difficult to check out whether the fish are enjoying
their warmer home.

My latest plan is to take two layers of Plexiglas and attach a couple of
inches of polystyrene to the edges to float the glazing off the water
allowing air exchange.
The alternatives seem to be a temporary greenhouse/polytunnel, or a wood
based construction.
The advantages of my idea seemed to be ease of construction, better
aesthetic and easiness of storage when not in use.
I am basically putting this idea on here for people to shoot the idea
down before I spend my hard earned cash on materials.
Peter

PS I should say that I live in the middle of the UK and the winters are
long but don't get very cold. In the 2 winter's I've had my pond has
had no more than half an inch of ice, however there is quite a bit of
wind chill which I think is what the bubblewrap cover has helped with.