What you said
If they are doing the work well, despite seeming not to concentrate, maybe you need to make it more challenging. If their lack of focus is manifesting itself in substandard work, just deal with it. Reasonably at first, but robustly if necessary, and with escalating sanctions. Top sets are more articulate and may be more skilful at complaining about you and getting parents on their side, so make sure you can justify any punishments.
There isn't a magic correlation between ability and behaviour; bright kids will still push the boundaries. You don't have to be an ogre - you can have fun with any class, but the top sets need rules just as much as the others. Would the pupils learn more if you were tighter on behaviour? If the answer is yes, step it up. You're not their friend, you're their teacher, so don't be afraid to act like it.
The expert view
Even the most able need to follow the regulations, which are designed to benefit all. Their chatting deters others from working and it reduces the amount they can achieve. It also reduces the effort they put into the task you have set. Whatever the motive for their misbehaviour, the outcome is the same. And if you have politely made it clear to them that their behaviour is unacceptable, and they continue to behave in the same way, then it deserves the same consequence as any disruptive behaviour: in this case, a short detention, working quietly.
Do not fret that this may deter them from participating in the future. You are the teacher, and you set the standard.
Tom Bennett is author of The Behaviour Guru and Not Quite a Teacher. Follow him on Twitter at @tesBehaviour. His latest book, Teacher, is out now. Post your questions at www.tes.co.ukbehaviour.