What counts as 'classical music'?

What do we actually mean when we talk about "classical music"? What is - or isn't it?

By its narrowest definition it's essentially mid-18th to early 19th century music and yet it's usually used to mean much much more. So how is classical music defined these days? Is it a walled garden of a very distinct style, or can it embrace all sorts of things? Does being played by an orchestra make something classical? Is film music classical? Are crossover artists classical? Is game music classical? Questions, and possibly some answers, with Tom Service, plus thoughts from composer Max Richter and writer Charlotte Higgins..