Thanks all, some wise words here!
We do already have a framework - ds starts h/w at X time, phone stays downstairs, he even has his own study with all his books and equipment right there so the temptations of x-box etc are out of the way. I check everything that has then been done when I get home from work and I text him to check he's cracking on before I get home. And, to be fair, he has improved somewhat.
The problem is organisation before he gets home. He has a planner in which he is supposed to write everything down; he often doesn't. There isn't any standard online set-up from school wrt to parents being able to check what h/w has been set every night - some teachers might put the info up, many won't. It is simply a case of he forgets to write it down, then forgets he has been given it even if I ask 'do you have philosophy/biology/maths h/w tonight?' and then it doesn't get done.
I ask him what he does in the lesson at the point when the teacher says ' right, tonight's h/w is...' but he just says, I don't know. Maddening.
The really frustrating thing is that in so many other ways he is proactive, self-starting and organised. If it's something he's interested in and wants to do, he's on it. This summer he organised his own job applications and interviews without any input from me at all, he's even got little 'freelance' paid writing jobs that he's sorted himself. He's independent and mature in so many ways except when it comes to subjects he dislikes. I'm hoping that all this does actually bode well for his future, but nevertheless he has to get decent grades in those subjects he dislikes before he can focus on all the stuff he does enjoy!