FB 851 then BA 712 at Heathrow
Sofia → London → Zurich
Timetable, London time
12:10FB 851 from Sofia at the gate
08:50BA 712 to Zurich leaves the gate
20 h 40 apart on the timetable, overnight
BA 712 leaves 20 h 40 after FB 851 arrives on the timetable: more than half a day, so not a connection we count.