423 Bus from Stand 7 outside arrivals, every 20 minutes until after midnight & £1.75 each.

Be aware there are two bath road Premier Inns but the bus passes both, if you are booked at the Premier Inn Named T5 its the second stop after leaving T5, the one named T2/3/Heathrow is about the 8th or 9th stop (Can't remember exactly which but someone will be along shortly to advise)