regarding advertising there havw been ads along the A13 and there are two large electronic billboards near Canary Wharf along the A13 so everyone driving that way into the city would have seen them which were regularly showing Flybe/Stobart ads for SEN. I've also seen ads on bus shelters around east London.
Heart has done a couple of competitions as seen here:
Win easyJet Flights From London Southend Airport - Heart Essex - Nov 2016
Win A European Break From London Southend Airport - Heart Essex - March 2017
Capital FM Glasgow has a competition to win flights
Win A Shopping Weekend In London - Capital Scotland
These radio competitions are likely to be repeatedly mentioned on air.
Local papers and websites have also all done competitions:
Win flights on one of Flybe's new routes to Dublin, Glasgow and Manchester from London Southend Airport | News | Yellow Advertiser
WIN FLIGHTS WITH FLYBE! | News | Visit Southend
WIN flights to one of these five destinations in today's Echo (From Echo)
https://www.southendunited.co.uk/new...s-with-flybe-/
So SEN is doing a fair bit of advertising and particularly for the Flybe routes.