Which floating plant

A single kind of floating plant isn't, unfortunately going to take out all the nutrients that are feeding the blanket weed, Mowgley. I don't know how much room you have in your pond, but I would suggest that you plant a number of marginal plants that will take up nutrients more substantially, and the bigger they grow the more nutrients they will take up.
I have planted marginal plants in my pond and have also integrated a bog garden into the pond which takes up even more nutrients. The result is that from Day One since I dug the pond in 1993 that it has remained crystal clear throughout. You have to really suppress the amount of nutrients that gets into the water before you have any effect and, in my opinion, only putting in a sufficient number of marginal plants will have an effect.

 

That's sounds like you're on the border line of success, Mowgley, in that if it stays clear during the winter. The fact that you're getting blanket weed in the warmer weather is obviously due to the pond being raised, as you said, and heats up more quickly than a conventional pond. The trick is to have enough marginal plants that will extract those nutrients to the point where the blanket weed can't grow.
I see your point about about a floating plants for shade. You could try Water Soldiers which rise to the surface of the pond during Spring, Summer and Autumn and then sink to the bottom during the Winter. They also propagate naturally and easily so they're easy to maintain. Bog Bean is another easy plant which although narrow in form has large-ish leaves to provide shade. Those two are hardy and will go through the Winter without a problem, other floaters such as Water Hyacinth, Frogbit, Fairy Moss, etc are tender and would need renewing every year. None of either groups will extract nutrients to a great degree I'm afraid though.

Pete's right about Barley Straw but the EU is due to legislate to ban the use of it due to being recently classified by them as a natural "boicide".........which is complete rubbish as it does no harm and works well.

Duckweed I would avoid like the Plague, as it is very invasive, becomes a nuisance and is very hard to get rid of!!!

 

I did have some duckweed in the pond a few years ago, brought in I think by birds, Pete, not a lot but it was there. As you know you can take out what you see but as each plant is small and independent of the others you're bound to miss some lurking in the corners or under other plant foliage. Anyway, it never got a hold on the pond as I have over planted on the margins, plus the integrated bog garden is fed directly by the pond so even more nutrients are taken out. I'm not sure but I have a suspicion that also the fish ate what duckweed they found as well.

The forthcoming ban on Barley Straw by the EU is just another idiotic ruling from people not knowing anything about gardening. People will find a way around it I'm sure, just as they have in the use of Yellow Sulphur.

 

If you go down the more planting route, Mowgley, you will need to be patient as you won't get an immediate result. The plants will need to settle in and start to use the nutrients in the water so it will take time, but you will see a difference as they begin to grow. I don't use a filter because I did research when making the pond on whether to use them or rely on nature to keep it clear. The late Great Geoff Hamilton [the best presenter Gardener's World ever had] did a program on water gardening and explained both routes but opted for using natural methods to keep the water clear. I didn't like the idea of using UV, having to clean out filter and all that Jazz and Geoff's advice seemed logical and simple. It worked completely for my pond and I have no regrets and have had no problems whatsoever since I dug out the pond. I do put on waders in early Spring and spend a hour cleaning out any dead foliage from the margins and the Bog garden, but that is a pleasure not a chore. I was planning on doing that this week but, unfortunately, had cut the ball of my thumb and a finger quite badly so I will keep out of the water until they heal.
I hope you don't have to go down the chemical route as it might have an effect on your fish and it's not a long term solution as the nutrients haven't gone away, so the problem is still there.

 

No, I don't disagree, Pete.
Let me explain what I did when creating a pond. First off I decided on a natural "wild life" pond rather than a formal feature pond. I dug it out to around 23' X 15' and down to a depth of 4' [in places] with a minimum depth of 3.5. I also sculpted out shelves all around 9" deep for the marginal plants, except for a place where it's shallow to allow creatures to get out.
All my marginals are in cheap plastic containers from Wilco's that look like miniature washing baskets costing a fraction of the price from the Water Plant Nurseries. I lined the baskets with Hessian, filled them with poor garden soil, put the marginal plants in, put gravel on the top to stop the soil floating out and have never had a problem. The Hessian eventually rotted away, after many years, but the roots of the plants held the soil in. The top of the baskets were around 2" below the level of the water, but that varies as does the level of the pond water.
Some people like to use the "Aquatic" compost in the belief that it is low in nutrients etc. But I was trying to keep the costs down, and besides Geoff Hamilton advised using garden soil,
and I followed his sound advice to the letter.
When I had dug out the pond and lined it, I built a wall of dry bricks [no mortar] in the pond on one side about 7' wide, and from end to end, to a level above where I thought the water would rise. I then filled between the dry brick wall and the near bank with garden soil again. Now when you think about it that should have given me problems regarding more nutrients in the soil because that was a lot of soil......but it didn't I planted up the bog garden, placed in the marginal plant baskets, put an inch layer of soil in the bottom and filled it up with water. After the initial murkiness caused by adding the water with a hose the pond was clear within 3 days and has remained so.
I have installed a small waterfall in the last 4 years but that has had no effect either way regarding colouring the water. I have fish in the pond and of course they add nutrient to the water from their excretions, but again, the marginal plants have taken that up. Nutrients come into the pond in many ways, dead foliage, insects, airborne dust, run offs from the garden [mine is at the bottom of a slope], so it is hard to stop the adding altogether. I'm not that sure that floating plants will take up enough nutrients to substantially reduce the nutrients. I have a mixture of oxygenating plants as well so I suppose a combination of marginals, floating, and deep water plants are the right way to go. Like all things, you have to do a bit of research.........and then go with your heart!!

 

Well, fish do add dramatically to nutrients in the water as they excrete all the time...and the more fish the more injection of nutrients. So that is a problem in itself but it is nice to have activity in the pond.
Marginal plants in the pond are Flag Iris, Japanese Iris, two varieties of Marsh Marigold, Skunk Cabbage, Acorus, Calla, Carex, Cyperus, Bullrush, Pickerel Weed, Reedmace, and Zentedeschia. They're all in baskets, although the Flag Iris have started to climb out of theirs The water in the pond seeps through the dry brick wall into the Bog Garden soil and feeds that all the time, so that is a great extractor of nutrients.

I don't think you can have enough marginals in the pond They attract and give cover to insects and wildlife, while hopefully keeping your pond in balance