I have just done this earlier today. Stansted Airport to Harlow Town by train, £6.30. Then take a bus from Harlow Town to Harlow bus station (£1.00), theh another bus (H1) towards Ongar (£2.20), get off at the roundabout after the motorway junction & then a 15 minute walk to the field. Total cost £9.50, a taxi is about £30.00 I recall.