If Heathrow was to be operated to its full capabilities i.e. no night curfew and full independent mixed mode, then the capacity could be increased by in excess of 40%.
Won't happen, of course, because the BAA would have to build 2 or 3 more terminals to cope with the extra passengers. Not to mention all the associated tube, Heathrow Express expansion that would be required. Might be a bit of a kerfuffle from the Cranford residents too, I guess. And I'm not even going to mention politicians.
It would be interesting to see if the costs of compulsory purchase of all the noise-affected houses around LHR would be greater than building "Ebbsfleet-in-the-Marsh International Airport". Probably take just as long.
Meanwhile in Amsterdam the other morning, in a three hour period whilst I was waiting for a connecting flight, there were flights to Edinburgh, Glasgow, Newcastle, Manchester, Birmingham, Bristol, East Midlands, Southampton, Aberdeen, Norwich, Cardiff, Luton, Liverpool, Leeds Bradford, Stansted, Durham Tees Valley, London City, London Gatwick and even a couple to Heathrow. Most were KLM Cityhopper B737-800s, even one of the Heathrows, so over 4,000 passengers. Now I know Amsterdam has its attractions but not on a sub-zero February morning. I would imagine more than 50% of those travelling, like me, were connecting because they couldn't get the flights they used to via Heathrow. This was a real surprise for me and just goes to show how much traffic is being lost by Heathrow - over 2,000 "customers" in just 3 hours and from just one European Airport.
On the beach
P.S. Our flight and the Glasgow flight were overbooked, too.
P.P.S. Forget the mixed-mode trial. It works. Just like it does everywhere else in the world. Hurry up and get on with implementing it before LHR becomes just an anachronism.