I've had a look at peak summer... those Sofia aircraft are being fairly well utilised!
Mon / Wed / Fri / Sun
06:00 Sofia - Luton 07:25 - 07:55 Luton - Zagreb 11:15 - 11:45 Zagreb - Luton 13:15 - 13:50 Luton - Bourgas 19:10 - 19:45 Bourgas - Luton 21:20 - 21:50 Luton - Sofia 02:55
Tue / Thu / Sat
Starts at Sofia 11:55 then to Luton for Varna or Bourgas, but still finishing up at 02:55
How on earth do they crew those sort of days?
Riga, Kiev and Prague to Luton all not yet on sale for peak summer, so maybe there is still to be some re-jigging of the schedules to cover the former two with a/c from the latter?