Character limit?

I'm adding some necessary code to the beginning and end of some cell contents in Format | Cells | Custom and I seem to be coming up against a character limit.

I'm using the phrase "!!<"@">!!" to add the code, but with some cell contents the result is ########### (but much longer), and I'm having to remove some of the text to make it work. The limit seems to be around 255 characters.

Does anyone know of a way round this?... I have just over 10,000 lines to apply this to!

Malcolm