If your offer reference number isn’t recognised, there are a few things you can try:
Make sure it’s the right letter — the reference number is unique to your appliance, so any other number won’t work
Check the number again — it’s usually in the top right of your letter, and has two or three letters or numbers and then seven numbers
Use the postcode on the letter — even if it’s wrong, you should copy it exactly (you can update it online afterwards)
Check similar numbers and letters — it’s easy to mistake the letter ‘O’ for the number ‘0’, or the letter ‘l’ for the number ‘1’
Try entering the number with and without a space