As I said, I can put any codes in the .CSV. I tried using quotes, it didn't make any difference, except when the text itself contained commas. Excel extracts the string from the quotes and then interprets it. Numeric strings are treated as numbers, and strings that start with a "+" are treated as mal-formed formulae. I can prevent the former by including a space before the close quote, but I couldn't come up with any way to stop the latter. The "=" is a general solution.
Being able to put any code in the CSV is implicit in my original statment that my application generates the CSV.