I'm not sure c3b is correct about the vast majority of SEN inbound pax not needing to travel onwards by train. That certainly wasn't my experience a couple of weeks ago when I arrived from WAT on a midweek evening flight. As I walked to the car park the two shuttle buses were taking on pax outside the terminal and there were a fair number of the 40-odd pax boarding.
On the point about LoCos not being interested in SEN "... through too much competition well established close by.", I feel that is the very reason one of them IS very interested, so as to offer an alternative that Ryanair cannot.