FB 851 then BA 634 at Heathrow
Sofia → London → Athens
Timetable, London time
16:45FB 851 from Sofia at the gate
10:15BA 634 to Athens leaves the gate
17 h 30 apart on the timetable, overnight
BA 634 leaves 17 h 30 after FB 851 arrives on the timetable: more than half a day, so not a connection we count.