Greetings, welcome to the board.
If you post an image of your £20 note, face and reverse, I can give you a fiarly accurate value on it.
Or if the picture is too big to post send it by email to me at the site email info 421 address.
10.9.28 is the first date, and is quite scarce (around 40+ in the census, I'd have to look up the exact number recorded). Prices start at around 1000 Euro for an intact note, and work upwards from there—a good note to have