Unquote #17 answer

The quote

My uncle always kept a marmalade sandwich in his hat, in case of emergency.

The puzzle as it was set

CB DQTWS LWMLBU XSVF L CLHCLWLKS ULQKMZTN ZQ NZU NLF, ZQ TLUS YR SCSHOSQTB.

The key

Each code letter stood for one real letter (code letter, then the real letter it hid):

L → AT → CK → DS → ER → FO → GN → HZ → IX → KW → LC → MQ → NY → OV → PH → RU → SF → TD → UM → WB → Y

The most frequent code letter was L (9 times), which stood for A. Short words, apostrophes and repeated letters are the usual way in: a one-letter word is almost always A or I, and THE is the commonest three-letter word in English.