I wouldn't worry about market demand, those people need to fly, period. The millions travelling to LTN to fly with Wizz might as well venture to Southend. LTN in its actual state is not a seamless operation to put it mildly hence the odd flights out of STN and BOH earlier and from LGW (and to lesser extent from BHX and BRS) recently. IMHO the short runway and the low category ILS of SEN (vs. the UK weather) are bigger risks than having enough passengers.