This is a common belief but in fact Heathrow is a 24 hour airport. There are a limited number of flights allowed during the "night hours" and the airlines prefer to use these slots after about 04.30 for long distance arrivals. You do sometimes see delayed flights arriving/departing in the small hours though. The ILS calibrator (a small turboprop) also operates at this time. Until a couple of years BMI operated a mail flight from Edinburgh that arrived around 01.00, but it has now been routed through Stansted.

I've been in Heathrow T1 at 01.00 from a delayed flight, there were plenty of others too, and the place was buzzing and many were sleeping in there overnight, public transport having stopped for the night. You can't throw people out under those circumstances.

Regarding the holding of the inbound Asian flights, the holding VOR, Lambourne, north-east of London, is visible from the back windows and garden of my own house, and indeed as I write this and it's a relatively clear morning I can see a couple going round. If I get up early I can see from the kitchen window how things are going with the early arrivers. The night slots finish at 06.00, so suddenly from about 05.50 onwards all the holding aircraft are drawn off and head across London. It's quite common for the first non-slot aircraft to touch down at Heathrow within 30 seconds after 06.00, with a solid line coming in behind at minute intervals. In fact it's one of the few times when both Heathrow runways are used for parallel landing movements.