Buses 350 (Terminal 5 – Hayes) and 423 (Terminal 5 – Hounslow) stop in Longford and at the Thistle Hotel. From the other direction the 423 can be caught from Bath Road on the northern perimeter as can the 81 (Hounslow – Slough). I think these buses would be free if getting on and off from within the Heathrow free travel area and all run fairly regularly during the day – about every 10 – 20 minutes.