LHR is a bit of a special case in that it has 'improved' so much over the game years. ATC was simply radio position reporting in the early days, ATC in the link a determining landing order and trying to maintain height or position clearances. Radar soon improved on that though it was some time before anything within ten to twenty miles of Heathrow had to have flight plans and I.D and Location Responders. The third runway finally disappeared in the 80's?, a SW-NE runway for lighter aircraft in strong winds. Since then the two parallel runways have been the sole provision, normally with one being generally landing, the other for take-off, as that helped with Taxi Clearances around the airport until T4 opened (Cargo only in those days).
While I lived between the flight paths for 27L/R there were regular occasions where planes alternately used L-R-L-R-L runways, maintaining the minimum 3 mile? separation on paths a mile apart by lagging by a mile or so. That was, and is, maximum arrival (and departure) rate.
1. Significance is the total movements have a maximum, but the number of arrivals varies opposite to the number of departures depending on demand. Peak demands were morning arrivals from the US(night flights) and East, and a team time peak (US chasing the evening flight and night flights to the East).
2. The total number of slots per hour varies over the years as methods and equipment improved. Some this was financed by charges to the airlines (ie per slot) to pay for upgrades so increase in slot numbers should come with an increase in slot charges.
3. The capacity of aircraft has also lead to big changes, T1 went a while back, T3 has now gone too, T2 and T5 now do the bulk of the Pax handling at a very considerable cost, offset to some extent by revenue raised by 'Shopping Opportunities' (Love Them / Hate Them?) but the Pax Numbers are also limited at most airports, as well as slots.
So, Is City Demand going to have the cost factors built in after capacity increases at airports, or the expansions actually driven by passenger through flow rather than historic demand?
And please, the above is written purely from my aged memory, by all means advise of any errors, or better data, I'll happily amend as appropriate.
As a foot note the fuel at Heathrow used to be Tank Stored where T4 is now, supplied mainly by underground pipework from Southampton's Fawley Oil Tanker Terminal and Refinery close to where the Flying Boats used to take-off for all points anywhere. I'm not sure how T4 affected the old tank farm.