Thank you VERY much Heathrow Tower for this interesting explanation.

Out of curiosity, during a normal day at LHR, what % of flights takes off using the actual slot that was assigned when logging the flight plan? (ie: how many flights have to request a new slot?)

Also... does it ever happen that a flight misses the slot time while being in a queue for take-off, taxiing to get to the runway? I wonder what would happen in such circumstance: flight X is trying to get to the runway, there are 10 more planes ahead and flight X physically can't get to the runway before such planes take off, flight X misses the slot and Eurocontrol assigns a new one 1 hour later... Would flight X have to return to its gate? In my experience this never happened; I am wondering what are the processes in place to avoid such occurrence.