LTNman
I wonder if you are confusing individual movement noise readings with the average over the 16 hour period. As far as I recall the situation regarding the fact of Stobarts having to purchase properties (or not) would be decided after one year(?) of ops from the extended runway once all the noise monitoring data was available. I haven't the time at present to look into the agreements in detail but perhaps someone else has them readily to hand.
I also seem to recall that there was a very small quota allowed within the movement cap for those aircraft which didn't meet the appropriate ICAO Chapter category for unrestricted movements. This was included to ensure that ATC Lasham's MRO facilities were not adversely affected.
Your inference that Stobarts may be concerned that a "big bill" will arise as a result of the planning consent requirements doesn't appear to be causing them to lose sleep.