.... Stanstead airport.

Can someone please advice the cheapest way to get two adults and a child
from Stanstead airport to central London and back again. Traveling in on a
Friday morning and returning on Monday evening. (9pm flight) I've had a
look at the Stanstead website and from what I can see the train prices are
almost as expensive as the easyjet flights I'm getting from N.Ireland!

TIA ... John

PS: First post here so greetings to you all from Northern Ireland.