Fares were only really APD+nothing during the week from STN if you booked in advance. If you tried booking the week or two beforehand or for Fri/Sat/Sun/Mon am they were considerably more. Stobart offered some ridiculously low fares and still couldn’t fill it. I got one for £9.27 or something similar.
For me the timings weren’t quite right which didn’t help. Last winter on Mondays it was actually the first flight northbound out of London, but that benefit was outdone by there being no return that evening and only mid-morning the next day which was useless for business. While it is now double daily, the evening one arrives in SEN quite late which puts me off when I can get back to STN much earlier and the morning southbound arrives too late to be of much use for Scottish business travellers.