Yes they are exempt so that clears that matter up as I have found this on the Southend website so it seems that Stobart said no.
Are night flights allowed?
Yes, but under strict conditions. Night flights are permitted by the Section 106 agreement between the airport and the Councils under strict conditions which place limits on the numbers and types of aircraft which can be flown. A “quota night period” is established between 2300 and 0630.Up to 120 movements may be permitted each month in this quota night period, however, specific rules apply to passenger flights.
The Airport may schedule up to 90 passenger flights in any month to arrive between 2300-2330. Late operating (arriving or departing) flights may also be allowed after 2330 under particular conditions – normally because not to allow them would cause passenger hardship. These are subject to review by the Airport Consultative Committee.