The hire company don't know who was driving, so the only person they can have sent it to is the hirer.
Is the notice addressed to the hirer, or to the hire company?
If the latter then it's a matter for the hire company to deal with as its addressed to them. What they should do (and the hirer should contact them to make sure they do), is provide the name of the hirer, who will receive a notice in his own name, which can then be appealed.