Hi wenders, that's correct. Your premiums won't change until your renewal, assuming you don't change your level of cover. Also, from reading [broken link removed], this increase only applies to customers of the plan called Level 2 Hospital.
One other thing to consider is that Aviva had a price increase back in March as well, so your renewal price next February will include both those increases and will be more than just the 9.5% in September. You should be notified of the new price a month before your renewal though.