More on Tripadvisor: the site also usually has a link to email a property - seen at the top of the page next to the link for the Hotel Website. And when I've emailed through that link (not to the LHR Hilton, but to other Hiltons), I've always received very prompt replies. And that's not surprising, since Tripadvisor has become in effect a top marketing tool for hotels, hotels seem to pay very close attention to emails sent through the TA portal.

Here's the link for the T5 LHR property:
http://www.tripadvisor.com/Hotel_Rev...e_England.html

And when you click on its "Email hotel" link you'll see the address shown is:
[email protected]

Here's the link for the T4 LHR property:
http://www.tripadvisor.com/Hotel_Rev...x_England.html

And the email shown there is:
[email protected]