LTN ought to be able to handle 480,000 ATMs on its two independent runways...
This 2002 report and forecast has been superceded by LTN's Project 2030 initiative and the estimates/proposals contained in the associated documentation.
There is no way - given the current environmental restrictions - that any new parallel runway at LTN could be operated simultaneously with the existing one, as major changes to minimum noise departure routes would be required.
The likelihood of any relaxation is remote, despite what some optimists might like to believe.
BTW befree, Eurostar will from 2007, operate into St Pancras International, not Kings Cross; and I agree that a significant number of runway slots could be released if travel by high-speed rail became a viable alternative to air travel.
Ironically, more people are using the UK railways than at any time since 1958 and much of the demand is coming from people taking advantage of the cheap 'APEX' fares now on offer over the trunk routes from the South and SE to the NW, NE and Scotland - cheap fares in response to the low-cost airline industry's head-on competition.