Topbunk - If aircraft operated to schedule then the congestion issues you refer to would be infrequent. Most busy airports in the UK are co-ordinated airports i.e they have declared capacity limits, managed by ACL, for both terminal passenger throughput and airfield infrastructure, including runways. It also takes into account surrounding airspace. Runway slots are allocated according to these declaration limits and at peak times a small average delay is built into the declaration figures. For many reasons flights often do not operate on time which results in too many aircraft either arriving/departing at times which exceed the declared runway, parking stand or airspace capacity limits resulting in longer than normal delays. Therefore a key factor in reducing fuel burn caused by congestion at airports is to improve punctuality and airports/airlines are constantly striving do do this, but with so many variables involved it's not easy to achieve.