LTNman
I'm impressed on your digging out the UK Rail 2015 conference details.
I see Glyn Jones comments as meaning that an airport without a current direct rail link might be reluctant to press for one as it would mean that 33% or more of their users would switch from arriving by car to rail. Such a "significant mode switch" would certainly cause a huge drop in car parking revenues at such airports. I didn't see his comments as relating specifically to SEN and its current situation.
I would be surprised if SEN were willing to trade the future provision of early and late trains (and its consequent negative impact on SEN's attractiveness as things stand today) just to retain car parking fees from those airport users who would otherwise use those early or late trains. Mind you, we are talking about SEN so I wouldn't entirely rule out such reasoning, however misguided you or I may think it to be. There was obviously some reason why Stobart coaches have not been employed to provide a London link outside of the current train timetable when there is an obvious demand for something of that sort.