I had a similar comedy of errors flight about 4 months ago out of LGW. The sequence was:

1. The gate was changed very shortly after the "go to gate xx" indication came up at T-50 - so most people headed to it. This gate was right up near the end of one of the piers
2. The new gate was one of the LGW-S bus gates, (gates 90-95 IIRC) right back near the main shopping area. Due to the original gate error, a lot of pax turned up late
3. We are now at the original departure time +10 and still at the gate waiting for he second bus to turn up. This turned up at T+15
4. Our aircraft is actually parked somewhere over near the North Terminal so quite a lengthy ride.
5. When boarding is complete the captain announces the APU is faulty and we will require an air start. This unit was already there and connected and they tried without success to start up the first engine. Lots of blokes in hi-viz outside fiddling with controls and scratching heads near the air start trolley
6. Captain announces the air start trolley has a faulty hose and cannot supply enough air, we must wait for a replacement.
7. After 10 more minutes, Captain announces the good news is the new start trolley has arrived, bad news is we have now missed our ATC slot.and we have permission to start engines in about 20 minutes. We are now some 1 hour past our original push-back time.
8. At last we are off at around T + 80 minutes and taxi out.
9. After a 15 minute or so taxi and hold in the queue we reach the active runway and without stopping promptly turn straight back off it and quickly come to a stop. Captain announces with some resignation "it really is not our day" and the reason we are still here is the inbound aircraft that landed just before reported debris on the runway so now an inspection must take place.
10. After a 10 minute further delay for the inspection we are finally off, some 110 minutes after scheduled time.