From memory, having had that issue a few times, I think you change the _config file encoding to UTF-8 (do that first) - ask if you don't know how to do that and then re-enter the £ sign.
I have tried this for myself and you can see the results here:
When I first opened the _config file (on linux), I saw a funny character (unknown byte basically). Then I saved the file as UTF-8, re-opened it, copied the euro sign from a random document (as I dont know how to produce it from the keyboard) and saved the file.
What you need to do is to open your editor (I used to use scite and now I use geany) and set the encoding for the file itself.
I have included an example of how I have done this.
If your editor does not allow you to do this then just download a small editor that can - just for this purpose. Just be careful also that in "FTP-ing" your file, it does not change encoding (not sure if that can happen - but I suspect it can).