I publish above a brief clip of John McDonnell speaking to camera about why he is standing. If you double click on it and enter YouTube you can find a number of policy statements by John as well.
In a comment to the Michael Meacher interview below, a John McDonnell supporter challenged my claim that I couldn't find a John McDonnell interview on YouTube. So I am obliged to Owen Jones from John McDonnell’s campaign for sending a link to a longer interview from John. Watch it here
And here is the interview with John McDonnell on Thursday from the BBC: