Event will take place on September 20, in Oxford
June values upped £530 month-to-month and £669 year-on-year
Average resale price up 5.2% from May 2012, but last month saw £78 drop from April
Research shows more than 50% of consumers want better coffee facilities and free Wi-Fi
Executive models and 4x4s rocket in value, as overall market enjoys 3.1% 12-month increase