Pobjoy,intercom in a Mark 111? your kidding! A bit like Children's Aid idiots at a local airfield, they tried to prosecute one of our pilots for teaching his son aerobatics without wearing a "Bone Dome", we pointed out to them that if a Pitts S2 went straight in that a Bone Dome would only ensure that the grey stuff would be easier to scoop up, then they wanted to lay charges for "endangering the life of a minor", under health and safety the whole world will, given time, grind to a halt. Never do I recall not being able to hear the back seat in any glider, mind you this could be that my instructors had to yell at me more than most students needed!