Unless you have a ludicrous amount of luggage, I wouldn't bother with the Hoppa - walk across Harlington Corner to the bus stop opposite the HI, then catch the 423 direct to T5 (12 mins travel time), or the 105/111/285/N9 to the Central Bus Station then transfer via Underground or Heathrow Express.

It's quicker than the Hoppa, more predictable, gives more options (particularly to the Central Bus Station) and free.