The theme is London Landmarks.
The answer ends in 'N'
The ditloid is: 4 C of B P S.
I can't get Buckingham Palace out of my mind but can't see anything that fits.
Graham...
Anyone else doing this? Would love a further clue to 16: Alps named after a Frenchman, set in stone. picture has a sign Val D'.. [rest hidden], and 18: Fred the gorilla was in this city's zoo and is...