giles - I get your point about growing traffic at LTN, but right now LTN has very little reason to offer cut price deals with airlines. That's not going to change until you see major building work underway at LTN. When you see at least 2 cement mixers and lots of people walking around in hard hats and heavy boots on site, then LTN and an airline might be actively engaged in talks about long term arrangements.
Airlines will happily take reduced fees arrangements, but there is very little that an airline can credibly offer LTN in return - there simply isn't the spare passenger capacity at LTN, meaning LTN can't derive any more revenue through per-passenger charges. Until that point, airport fees at LTN should be set to maximise revenue, so it makes sense for LTN to act in the same way as LHR - namely charge the maximum price possible without the number of passengers being too much below the airport's capacity.