Well, SEN naturally does have its own catchment area which I would define as those who live within the same travelling time from SEN as they do from the other London area airports (including time spent being bussed from their remote car parks), plus those who wish to avoid the ever increasing hassle at those other airports. That should amount to several million potential customers I would have thought. The proposed rail station only serves to increase that market even further surely? As Barnaby the Bear says, "the train would be the best bet".
I'm not sure that flying on a Q400 as opposed to a B737/A319 features particularly highly on the average person's priorities - unless there is a choice of competing carriers from the departure airport itself. This is exactly RAL's point on this - Flybe (or whoever) isn't at risk of establishing a Q400 route only to find that Ryanair/Easyjet muscle-in with a 737/A319.
I agree entirely with your suggestion that routes to 'niche' destinations such as Caen should have good potential from SEN as that would automatically attract pax from other London airport catchments which do not serve those destinations at all.