This would be only the third hotel at Heathrow, iirc, that does not cost to get to.
Hilton Terminal 4 has a free shuttle and is adjacent (via a covered walkway) to Terminal 4; Sofitel at Terminal 5, and now this one. Otherwise, getting there requires the National Express Heathrow Hotel Hoppa
bus - purchased on board, £4.50 single, £8.00 return. You could take a black cab, but unfortunately BAA and the black taxi drivers have an agreement - minimum ride one way anywhere from a LHR terminal is now £35.00 (USD $56.00!), though you can usually find a minicab back from a hotel for considerably less.