I've spoken to a legal expert at the FSB and he reckons that sending out an email newsletter from the new group to the existing subscribers of the previously independent hotel should be fine.
This is on the provision that in the email the reason that they are being contacted is made quite clear and linked to the original act of signing up. An opportunity should always be given for them to unsubscribe should they wish, but this has been standard practice anyway.
For example, the group could tell subscribers that they are being sent the email as they originally signed up to the 'hotel A' mailing list and 'hotel A' is now a member of 'group B'. This makes a link between the email and the original act of signing up. What the subscriber does now is up to them.
Just sending email newsletters to these subscribers without carrying out this intermediate step is probably in breach.
The FSB guy reckoned that if there had been a clause like you quoted above then you are fine. If there wasn't (which there wasn't!) then this alternative is the way to go. So, although you don't claim to be an expert you were pretty close to the mark with your response!