The fruitlets look as if they could have been attacked by plum sawfly - unless you have lots of plum trees, or have had trouble with this for the last few years, don't worry about it. What I'm not sure of is what's caused the web on the leaves - usually these are moths of some kind, or tent caterpillars, but can't find any info regarding plums specifically.

Thank You Bamboo. So you don't think that little yellow thing with the 2 black dots on it,on the leaf beside the little web mound, has anything to do with it? I wondered if this could be a mite of some sort and wondered if it could be contributing to any of the damage -

I can see it, but have no idea what it is and not present so can't get my magnifying glass out (!), and it may or may not be related to the webbing. If the tree is small enough, you could spray with a general insecticide under and on top of the leaves, (check the Provado range, see what infestations the various ones cover) but doing that now isn't a great idea if you want to eat any fruits, so I'd wait till the fruits are gone.

Hi again, I did some detective work and I've discovered it is TSSM (the two spot spider mite) Bamboo. I've written all the info. about this mite on a blog. It's not something I had ever heard about before. I've just this minute jet washed the tree as a start anyway, and removed as many of the leaves with the webbing as I could see.

Well you learn something every day, never heard of the two spot spider mite!