Is that his house number? It may be the bicycle polo episode where he leaves home in his Johdpurs ?
It's the number of the bus that Foggy was on in The Man From Oswestry.
I was going to say the number 11 bus. It's odd because when the bus arrives it is the number 11 but when Foggy gets off it's the number 14. What we do know from Cyril's letter is that it's the 11 o'clock bus so that's going to be my answer, The Man From Oswestry and the 11 o'clock bus.