Thank you for all the additional feedback; it's very helpful to see how other people see this. All the comments from people at my son's end of the age spectrum are so helpful for getting an fresh insight from the Youth angle. What's clear is that almost invariably Young Adults At Home (shall we call them YAAHs?) are expected to and do contribute in a range of ways. And that domestic quality of life seems generally fairly harmonious for both generations. Which confirms what I knew, really: that my sons' behaviour in recent years really is beyond acceptable by common standards - so it's not just me.
To answer some of the questions:
Yes, I do work, part time, and crucially - from this home. Which makes the need for cooperative help from young people that much greater. People come here to see me, so the need for presentable common areas is paramount - often leaving me forced to pick up after everyone, in these recent circumstances.
From their early years, I made sure my children did 'their bit', increasing gradually as they got older. As I said in an earlier post - I always had in mind their possible future partners, and was keen that the boys would become competent at all aspects of housework, cooking, cleaning etc. I really did think about all these things very carefully, and knew I had to work hard to make it happen, as their father made it clear he wasn't going to back me up on anything from when they were tiny. He 'walked' when they were between 4 and 8.
They were fine when younger. It's only in the last few years that things have become really difficult. Basically since my oldest came back from uni and spent a year unemployed/trying to find a job that suited his - all too high - expectations. He eventually 'compromised' and took a job 'beneath' him, but during that initial year his behaviour deteriorated. It hasn't recovered. Then his brother came out of a long, demanding training scheme and wanted to catch up on missed party time - right here, in this house, with his mates. I like young people around, and they have nice friends. But partying midweek till small hours, leaving beer cans everywhere? Asking for moderation gave no results over time, so I finally had to ban drinking at home except Fri/Sat. They are still outraged.
I think poster MaryZ said she may have to accept living in a student house. I too have been feeling just like 'it's a student house with annoying adults in it' - but my take is that this is unacceptable (plus I work here). So it's not student digs and the conflict arises because these sons try to live as if it were.
I don't mind being the only one who notices the loo rolls running down etc. I don't mind having to be 'The Manager' and delegating. But if the response is disrespect, lack of co-operation, and reasonable asks become stressful for me, then things have clearly gone to far.
After reading everyone's comments, it is even more opaque to me why it's happened like this.
UPDATE:
We asked the oldest son to move out last night. It was a bitter conversation, and my son showed all the worst traits, being extremely rude and sarcastic particularly to my partner. Plenty of 'family dynamic' stuff being played out there, clearly. In spite of his meanness, we gave him a month to find a place. On condition he switches off the sarcasm and rudeness, and continues to do his bit. But this morning, when only i was here, he carried on in the same toxic vein. I think he either can't help himself, or he's trying to drive me/us to instant ejection (perhaps so he can maintain a position of 'victim'). So now I'm weeping into my porridge over the loss of the sweet boy this son once was.
So sad. But thanks for all the comments, because it strengthened my resolve and confirmed what I really already knew.