I don't use T5, but 2 things could possibly help you:
1. you can take any free public bus to Heathrow Central (Bath Road is a free bus zone - any bus going from Bath Road to Heathrow Central is for free) and then catch a Heathrow Express train from T1/2/3 to T5, which is also for free IIRC.
2. you can use journey planner at
http://www.tfl.gov.uk/modalpages/2605.aspx - I am not sure if there are any direct busses from Bath Road to T5, but please check this planner.