Do you still have a copy of the letter?
If you don't, just generate one.
UPC don't seem to have retained records from the NTL days. Our account is in my wife's name (for ID purposes) with me authorised to correspond with UPC. I recently called them and they refused to speak with me on the basis that they no longer had an authorisation letter on file "as it related to the NTL days".
Face them down, call them, send them a copy of the letter. Just don't be fooled into paying the €200. They are a very difficult company to deal with.