BA 1399 then BA 135 at Heathrow
Belfast City → London → Mumbai
Timetable, London time
16:25BA 1399 from Belfast City at the gate
15:05BA 135 to Mumbai leaves the gate
22 h 40 apart on the timetable, overnight
BA 135 leaves 22 h 40 after BA 1399 arrives on the timetable: more than half a day, so not a connection we count.