Yes ... I recall BAF with their significant fleet of VC8's, HPR7's and Shorts Sheds being based and actively busy at SEN, then there were the evenings when much of the country, including STN & NWI would be fogged out, SEN would be open for Air UK to divert much of their fleet in to ... Air UK never bothered to divert to LTN that much!
For many a day SEN has been a maintenance facility, Heavylift would put their Belfast's and Airbus's in to there, also a maintenance facility for adhoc geriatric jets such as B707's & B727's, before those days, my ex boss, the much loved and famous Sir Freddie Laker based himself and his Aviation Traders at SEN, such aircraft as the 'Accountant' were built at SEN, later on there was National Airways based there and thereafter Flightline and BWA, SEN has an interesting history to say the least.
And regarding a runway extension, for many a year the talk has been "What are we going to do with that church?".