FB 851 then BA 636 at Heathrow
Sofia → London → Athens
Timetable, London time
16:45FB 851 from Sofia at the gate
13:25BA 636 to Athens leaves the gate
20 h 40 apart on the timetable, overnight
BA 636 leaves 20 h 40 after FB 851 arrives on the timetable: more than half a day, so not a connection we count.