I just got this on an email from Amex:
Thank you for sending the requested documents. We have now forwarded the documents to the Merchant for more details and as we previously ensured you were not liable to pay this amount while we investigated, we will now credit the amount and you will see this on your upcoming statement. If the Merchant provides information supporting the transaction, we may rebill your account. If this happens, we will update you.
I also attached the T&Cs from March and a cover note explaining TravelUp may respond with copies of their T&Cs but to take the March ones into consideration since that is what I signed up to (thanks again to
joequimby for providing this).
So hopefully TravelUp won't respond as I already had the T&Cs part covered, but let's see.... For now Amex have credited the amount back to me and closed the dispute.