I'm sorry, Steragram! I saw the question, and the black insects could be blackfly, but the white webbing has me stumped! I thought that I would wait until later in the day, to see if someone more familiar with the UK's pests could answer. Is there any chance of a close-up portrait of the pests?

Dont be sad I am sure its treatable and I think its great that the site does not answer if they dont know.
I thought the same as Tuqb and was waiting with interest to see what others said. I thought by your description you had Black fly and maybe because of the white webbing white fly or possibly Ermine moth? But one can only guess without a photo.

Thank you so much folks, I was feeling unwanted!lol.I will have a go at a photo but the pests are so small I don't know if they will show. I'll try a macro and see what happens.

A photo of the white webbing would be the best thing

I've put some on my photos - couldn't see how to add them here. Now I can see a magnified pic there are clearly aphids, and the white stuff isn't webbing, its sort of lumpy. Most of them seem to have gone now, but as this happened last year and the year before too, any ideas on how I can prevent it next year? The poor tree is looking very sad as you can see.

I dont like to spray insectacide on any thing I eat ST, so I cannot recommend one, I spray with a solution of washing up liquid when I see an infestation it seems to work for me.

After a lot of reading, I have found that that white fuzz is something that the plum leaf itself produces in response to plant-hormone-like secretions from the aphids. The fuzz, along with the curling of the leaf edges, helps to hide and protect aphids from their predators. About the only things that you could do in the organic line is to spray with organic washing up liquid, as Drc recommends, or encourage the aphids predators by giving the tree a weekly hard spray of water, and maybe trying a spray or two of baker's yeast, 1/2 teaspoon per gallon, to attract lacewings.

How very interesting Tug - it is so kind of you to go to the trouble of researching it for me. Isn't nature amazing? I always use an eco washing up liquid so that should be easy enough Drc, but I'm horribly afraid its a bit too late this year. Will give it a go anyway. I don't ever remember seeing any lacewings in the garden but no harm in trying the yeast as well.

I only wish I had thought of asking Goy earlier in the year.Everyone is so kind.

Well, every bit helps! If the tree can use the remaining leaves, without the aphids sucking out all the sugar, it will be happier next year. An application of seaweed this fall, just before the leaves turn, will help it to resist aphids next year, too.

What do I do with the seaweed? I guess I could collect some from the beach (though there often isn't much there)

Well, you should be able to get some ground up dry seaweed--often sold as kelp meal--or liquid seaweed extract, from the local nursery or garden center. Just follow the label directions.

Being by birth a Yorkshire woman I am reluctant to buy something I can collect for free! I will take a wild guess - collect it, wash the salt off, leave it to dry, crush it. I will need to look at a label to discover whether to spread it round the roots or spray it on! I think it is actually against the law here to remove stuff from beaches, but maybe that only applies to sand and gravel rather than seaweed!

I'm sorry! I forgot that you were near the coast! Even if you weren't, it probably wouldn't be as much of a project for you to holiday at the shore, as it is for me! You've pretty much got the proces down, except I would put it on dry--it's a little hard to dissolve, and it works better dry. Spraying it on would work even better as a repellent, but only for a week or so.

So I just pile it on as a mulch? Gosh I would miss the sea if I lived in the desert! I guess from your knowledge of our plants that you used to live in the UK? Did you have to move for your job?

No, I was born and raised in the desert, but I have been a plant freak since I was ten. If you work at it long enough, you can get a fair amount of general knowledge about how plants, and their pests, operate.

Must be quite a challenge growing plants in such an arid place. I used to be a cactus collector at one time but only have one or two now.

It's actually not that hard, but my water bill is horrific! }: 0
You also need to pick the right kinds of plants--Clematis are pretty hard here, but Bougainvilleas are easy.

Well, horses for courses - the bougainvillea wouldn't last long here. Even Wales has been drier than usual this year, but not exactly desert standard!

Oh! That's right! Wales...not New South Wales!! : D

Spot on... this is the old West Wales. Iechyd da! (good health)