[clivehawkins]

I had a PET class with a similar problem, although not as severe as you've described.

I gave them a particularly hard mock which 'the chatters' failed pretty miserably. I know that we shouldn't demoralize the students but in this case I felt it was justified. The result scared the hell out of them and each time they slipped back into 'chatting' in Italian I reminded them of how much work we needed to do to pass the exam. I also reminded them how much the course was costing as well as the enrolment fee for the exam - all of which was being paid by the parents.

It did the trick and they all passed. Maybe I was just lucky that time but you could give it a try.