Found the reason buried on the confusing CAA site. London Airspace Management Program introducing RNAV procedures which will be flown by the 70% of aircraft using LCY; the other 30% will follow the same procedure using existing aids. RNAV equipment will be mandatory from 2017
Quote from the decision:
The justifications presented by NATS for the LAMP Phase 1A proposals are that it will modernise airspace structure, improve the operational efficiency of the airspace providing capacity for the future, minimise future delay, improve the environmental performance of the airspace, reduce average CO2 per flight and reduce the incidence of low level overflight of populated areas
I guess it will settle down once the airlines have built the extra time required into the timetable, but whilst I am sure everything will "flow" along nicely once implemented, adding to aircraft track miles won't do anything to reduce CO2...