1

http://bit.ly/Yr5G4C Allow at least 4.5 hrs between landing and take off, more if the transfer is between 6am to 9am and 4pm to 7pm.

2

National express the most convenient quickest possibly train into central london, tube to paddington then heathrow express, or google airport taxis and check out the price for a pre-booked depending on how many of you could be cheaper.

3

Using the train:

Get the bus from LTN terminal to Luton Airport Parkway station (10 mins, but up to 30 in the morning rush).
Fast train to St.Pancras (min 25, max 40 mins). Fastest train is nonstop with London Midland. More frequent is with First Capital Connect, but slightly slower
Picadily Line (dark blue underground line) direct from St.Pancras to Heathrow takes approx 55 mins, and go every few minutes.

So, allowing for perfect connections, minimum 1 hour 45 mins.

A bit faster, but much more expensive would be to take the tube from St.Pancras to Paddington (10 mins), then the Heathrow Express train (15 mins).

4

The easiest and fastest route is to take a National Express bus directly from Luton to Heathrow. For detailed infomation see this page on the London Toolkit site.

5

But first you have to wait for it, and they go at fairly random times, about every 15 mins, and often connect poorly with the train service at the station, even though the trains and buses are run by the same company. Then there is a whole fuss about whether you have to pay on the bus, (if you have a FCC or unrestricted national rail ticket to Luton Airport PWY you don't) which makes it take ages to load the passengers. The walk at the station is bizarrely long as the footbridge level is very high above road level. The useful trains run at 15 min intervals So, realistically, even when there is no traffic congestion, you get a train about 35 mins after you walk out of the airport terminal building. Poor organisation makes what ought to be a short and quick road connection of only 2km about as annoying as is possible.

Then, on those trains that run every 15 mins, it takes 35 mins to KXSP. Then its quite a long walk to the tube through this megastation, allow 15 mins. Then the Picc train doesn't come immediately, and usually manages to wiffle about on the way, so it takes about an hour.

So, in the real world, this takes something around about 2h30.

6

Well, I did say " So, allowing for perfect connections, minimum 1 hour 45 mins."