The Hilton T5 is not on a free bus route, it's not even in London (the free Heathrow buses are run by Transport for London). I normally eat there if I'm staying at the Hilton and would recommend it. I met Cyrus Todiwala in the restaurant so he certainly does spend some time there and is happy to talk to customers.

There's numerous ways of getting there depending on your budget. Don't get a black cab though as it's outside of the Greater London boundary their metered fares don't apply they have the option of refusing the job or quoting you a fare (which is usually expensive).

The hotel can order a cab for you (fixed rate) or there's always Uber. Other options include walking if the weather is nice, take a Hotel Hoppa bus (£5 each last time I checked) or one of the local buses that pass close (not sure of the fare but likely cheaper than the Hoppa).