An army of lawyers, consultants, town hall officials, planners, noise pollution experts and local residents have been giving evidence to the inquiry, which has now been running for 60 days more than the previous record-holder - that into the construction of the Sizewell B nuclear reactor. That investigation gave the all-clear, and the plant opened in 1995.
By the time the T5 inquiry ends, it will have cost around pounds 100m. The investigation, which started on 16 May, 1995, was originally meant to last 18 months, but inquiry inspector Roy Vandermeer QC is expected to hold hearings until the end of this year, with his report not due until after 2000.
The flight of fancy that is Terminal 5 was devised by Heathrow's owner, the British Airports Authority. The company can only start work on the pounds 1.8bn project - designed to increase Heathrow's capacity to 80 million passengers a year - after ministers give the terminal the go-ahead.
But the real result of the hearings may be the ditching of the public inquiry system itself. Last week, in a private meeting with Labour MPs, environment minister Richard Caborn singled out the T5 inquiry as an example of the urgent need for reform of the planning laws.
TO MARK day 400 of the inquiry, here are 10 things you didn't know about the T5 farce.
1) The biggest winner since the inquiry started is the Ramada Hotel - a four-star establishment on the edge of Heathrow, where the inquiry is being held. The biggest losers have been its guests - the hotel's swimming pool has been converted into a hall and about 50 bedrooms are in use as offices.
2) Some of the barristers involved are being paid an estimated pounds 2,000 a day. Latest estimates reckon the inquiry has cost the central Government pounds 8m and local government pounds 7m. BAA has already spent pounds 41m.
3) The airport operator may be ruing the day it dreamt up the new terminal. The company says it is set to cost it pounds 200m by the time a decision is made.
4) More than 650 people have given evidence to Roy Vandermeer, spouting an estimated 25 million words.
5) The inquiry is nothing if not thorough. More than 50,000 questions have been asked of the witnesses appearing. At least 4,000 documents have been handed in.
6) BAA's photocopier in the Ramada hotel is, according to its manufacturer, the second most used in the country - with more than one million copies taken.
7) The inquiry into London's new terminal has seen inspectors take trips to Amsterdam, Paris, Frankfurt, Chicago and Singapore.
8) BAA say 27,000 people support the new terminal. The development's opponents say about the same number of people oppose it.
9) Mr Vandermeer is helped by nine planning inspectors. So many have been recruited, minister Richard Caborn explained recently, "in case some of them die before it's over".
10) Terminal 5 won't be ready until at least 2005.Reuse content