There is always the HEX (train) linking the Central Area and T4, use that for free also. You are right, the free bus network is not very well publisised. I often chuckle seeing people swipe their "oyster cards" on the LT buses for airport-airport internal journeys. they have deducted 80p when the journey is otherwise free. Works the other way of course when people override say to Kingston on the 285, and stay on after Hatton Cross.