Dorchester third night free applies 45 Park Lane and, of course, The Dorchester.
http://www.flyertalk.com/forum/luxur...nov-12apr.html

Don't forget to look at Virtuoso etc... Promotions. Dorchester and Claridge's promotion could also be used by TA to negotiate same at other properties.

I would look at Covent Garden Hotel, FS Park Lane, The Connaught, The Goring, etc... You don't have to limit your options to what Amex is offering.