Glad you’re finding the script useful!
I don’t think you’re doing anything wrong.
Extract Pages plays it safe with file names, and removes any characters that could potentially cause problems. The £ is in the list, so it’s being removed.
I found a list of legal filename characters somewhere on the Internet, and used that to filter out any illegal ones.
Not much that can be done at this point without modifying the script, I’m afraid. I could do that if it’s very important for you to have the pound sign there, but if anything goes wrong, you’re on your own!