Avoid the airport! Don't fly from Heathrow to Charles de Gaule, if you can take EUROSTAR trains, you only have to change trains once. From London, the high speed rail takes you to Gare du Nord (2 hrs 15 minutes), Paris' Northern Station. From Gare du Nord take the Metro Line 4 or RER B or D to Chatelet Les Halles. Then transfer to the RER A 4 to Marne La Vallee-Chessy. The journey takes about one hour.
You're still saving time compared to going from Central London out to Heathrow, being an hour plus early for the flight, then taking the train into Paris from Charles de Gaulle.
However if you insist on taking a flight, look into the direct shuttle bus from the airport to Disney.
From Gare du Nord, the Métro/RER combination is:
M4 to Les Halles; Transfer to RER A4 to Marne la Vallée
Definitely get Eurostar. There is a daily direct service to DLP direct from London but timing isn't great so either go via paris as californiaAdventurer says or see if you can get a connecting TGV or Thalys train from Lille which will be easier & avoid Paris.
Book a Francilien ticket 5 days in advance for savings either through the French DLP website (use google translate if you don't speak French) or FNAC and print your tickets at home to save time.
Depending on what you've seen in other Disney parks consider doing the different or unique attractions at the resort. With one dat at Parc Disneyland do Pirates, Phantom Manor, Space Mnt & Big Thunder (if you like coasters) which are all different to their stateside versions.