No hotel physically in terminal 5 itself.
Nearest option is the Sofitel which is adjacent to the terminal and linked by a covered walkway, 5 minute walk. This is the only hotel option in walking distance.
Other options require a drive/taxi, train or bus transfer except The Thistle which can be accessed via the T5 business car park automated pods.
The "Hilton Terminal 5" isn't actually at T5 but a 10 minute drive away on the other side of the M25.