I really like that suggestion, Dilly. Not so much for H - his ego is too delicate to use humour with most of the time, especially around this issue. But it could work with Eldest.
I think I also need to STFU more and get a sense of proportion. Eldest is stroppy and obstructive and often whingey and lazy and moany. But he's also doing very well at school, he doesn't take drugs or smoke, there's no risky sexual behaviour (that I know about) and while he has friends and socialises, I always know where he is and when he's due back. He can be extremely disrespectful to both me and H - more so with H - but I have the feeling that it is within the realms of normal and it's my / our reactions to it that need to be finessed. So - his behaviour isn't acceptable but it isn't hugely worrying in the grand scheme of things. And perhaps he's just going to have to learn to deal with the fact that H isn't going to respond with calm and care and understanding - lots of people wouldn't.
I suspect what's behind this problem is the fact that H over-reacts, and I over-react to H over-reacting, and I pre-over-react if they're going to be in the same room together and get uptight, or try to match make or manage or control things. And H pre-over-reacts to my over-reaction - I think he gets tense and worried thinking that I am going to get upset before Eldest has done or said anything. I probably have a greater influence over what happens in my house than I imagine, and the way through it might be humour, calmness and STFU.
I am away next week, leaving them all together and I am very anxious about it. I wanted to sit down with H and talk about parenting and Eldest, but perhaps the better route would be to just leave some food in the freezer and let them fight it out together.
