Which episode? Misspelled sign

Anyone know which episode this is from?

Arkwright puts a sign outside about a special offer. Granville sees it later and says that's not how you spell ????? (can't remember - maybe apples?)

Arkwright says it doesn't matter. Those who don't know any better, will come into the shop anyway. The irritating people who get worked up about bad spelling will feel compelled to come into the shop to put him right and then he can sell them something.

A cunning plan indeed!

But which episode is it ? Someone please put me out of my misery.