I've run into the 9-night limitation, too. I think I just made two separate online reservations then advised the check-in person upon arrival of what I had done so we wouldn't have to change rooms. You can try calling, but getting a Premier Inn to answer the phone is a serious challenge. Some locations may be better than others, but I know I called once and received a voicemail message something like this: "We're very busy right now. Goodbye." No option to remain on hold or to leave a message, and they probably wouldn't have called back even if I had been able to leave a message. At another location my travel mate needed to make a telephone call because she had lost her purse. The hotel generously offered to let her use the hotel phone. It was a mobile phone and the battery was dead. We had a good laugh about that, because it was just so typical of Premier Inn.
In your case, calling the central corporate phone number for reservations (if you can find one) really should work. Maybe.
I've had some success getting answers to (simple) questions via email, but some hotels didn't respond, and finding the email addresses was a challenge itself. What worked best for me a few years ago was Googling for the Facebook page of a specific Premier Inn and looking for an email address there.
Honestly, the Premier Inn hotels seem well maintained, and I'm happy to stay in them, but they keep their rates low by skimping on customer-contact expenses.