Refusal to reissue damaged credit note

I purchased a pair of shoes last year. They did not fit and I returned them. I got a credit note on a till slip. I kept it in my wallet. My wallet got wet and the till slip is now unreadable. <br> <br> Customer service are refusing to re-issue the note even though they can see my records on their system. They claim it is just my bad luck and they are not responsible.<br> <br> This is effectively theft. We live in a digital age.