c3b
It wasn't just Channel Airways at SEN in those days - there were many European operators such as Condor, Braathens, Schreiner and Transair to name but a few. Luton certainly attracted a number of such operators who would naturally have considered SEN - as their equivalents seem now to be doing - but for the runway length.
The simple fact was that required distances increased with legislation and SEN's already marginal runway was no longer sufficient. I doubt that it was a complete coincidence that the demise of SEN coincided with the growth at LTN. I agree that a number preferred to move to STN but I can assure you that in those days, rightly or wrongly, LTN was a dirty word among the SEN fraternity. Come to think of it, perhaps with SEN being a dirty word at LTN for some people now, what goes around comes around.