I once had a £20 note like this and made a mess of trying to repair it. I sent it off to Bank of England place in Leeds I think as I was advised by my bank that this is what I had to do. I got a letter back saying I'd get no money back as it was a forgery
well believe it or not - it was a polymer note, I shouldn't have accepted it but it was the local chippie and the assistant was wearing PPE gear so I was a bit nervous