I have installed the certificate and it is recognised as a certificate issued by LE. But when I try to copy and paste the LE Key I get a message that “The key does not match the certificate”
Are you sure you are using the right key?. If you didn’t upload your own key nor csr, zerossl generates them for you, indeed, if you have download all the files, you shoudl have 4 files:
You cert is domain-crt.txt and the key you need to use is domain-key.txt maybe you are trying to upload the account-key.txt instead of domain-key.txt…?.
You are right (of course) I was trying to upload account-key.txt not domain-key.txt! Can I recover the domain-key or will I have to go back to the beginning and do the whole thing again?