sggolf Alternatively, rather than staying at LHR and travelling into London for the day/ evening, you might consider staying in London and going back to LHR the follwoing morning, if your travel plans permit and if you don't have too many bags.

Most LHR hotels are outside the airport and require a separate, paid for bus connection to/from the airport. Staying at LHR would therefore mean:

LHR (paid bus) to Hotel to check in drop bags
Hotel (paid bus) to LHR to catch tube or HEX
LHR (tube or HEX) to London
London (tube or HEX) to LHR
LHR (paid bus) to Hotel to sleep
Hotel (paid bus) to LHR to check in and depart.

You will give yourself more time to enjoy London if you stay in town and probably get as much time to sleep as well - the trip back to LHR in the morning probably won't take much longer than getting to LHR from a LHR hotel would.