Here's a link to the planning application form that was submitted by Howard De Walden Estates.
http://idoxpa.westminster.gov.uk/online-applications/files/B563B8EE96196EE101D395B3905D2B0F/pdf/09_09712_FULL--1764352.pdf. Scrolling down Sam Smiths are described at the time of application as the holder of a lease with at least 7 years left to run, so I wonder if the lease was terminated by mutual consent, or maybe even renegotiated with SS contributing to the development.
09/09712/FULL | Use of first floor of 33 New Cavendish Street to provide relocated Class A4 public house kitchen and office accommodation in connection with retained Class A4 pub on ground and first floors of 35 New Cavendish Street, use of second to fourth floors of Nos. 33-35 to provide 1 No. 1-bedroom flat and 1 No. 3-bedroom flat, and alterations to rear elevation including replacement of windows, removal of small rear extension and installation of kitchen extract duct equipment in connection with relocated kitchen of public house (part of land use swap with 5 and 20 Bentinck Street). | 33-35 New Cavendish Street London W1G 9TS
