I can second that... STN does have the highest proportion of passengers using public transport of any UK airport. I think it's about 40%. I can't be bothered at the moment to find a link to be brutally honest, but should be quite easy to find somewhere.
Actually you are way out with an under estimate but you are right that Stansted does indeed lead the way.
London airports have the highest proportion of passengers travelling by public transport with Stansted the highest (50 per cent) followed by London City (46 per cent) and Luton with the lowest (29 per cent).
Public or private transport, how do you get to the airport? Latest CAA passenger survey highlights stark regional variations | UK Civil Aviation Authority