In the email from Hampton, 800+ applied for 11+ and 35% were invited for an interview.
DS was interviewed by 2 teachers, 15 minutes each, for math and English. A few math questions are given. Read a paragraph and be asked a few questions. Plus some general questions: which schools did you apply to, which is your favorite (Hampton for sure), how did you prepare for the exam, what are your favorite sports (and what team did you play), what did you do in the Christmas term break etc.
About 20 boys in each batch and when they went to interview, parents are brought to a lecture theatre and the HM and admission director gave the briefing. He specifically mentioned that, unlike some other schools, they won't ask the boys about current affairs. That means for some other schools, suppose you need to prepare.
They didn't interview parents but when the boys were brought back to the lecture theatre after the interview and reunited with the parents, they did seem to be scanning and looking for something and resting their eyes for more than a few seconds on each family. But then maybe I am thinking too much.
Hope the above help 11+ preparation next year.