I'll chuck my 'two pennies' worth in...
Hapag Express at STN...
Follow the logic... (although I know that the Hapag Express Aircraft arent they're own)...
Hapag are part of TUI, which own Britannia.
Britannia is based at LTN.
Britannia Engineering is based at LTN.
Britannia Engineering are contracted to service Hapag's 737's.
Hapag are paying (Germania?) to operate the Aircraft.
Logic say's... Lets use STN, not have any engineering cover (that could have been suppiled by one of our own in-house companies!) therefore keeping Aircraft servicable if the go tech in London, and at the same time re-charging Germania some of the money for Engineering support that they are paying towards the Aircraft!!!
It's the same old story - BAA's pockets are a lot deeper than LLA's!