Do these requirements apply to small water heaters such as water heaters of less than 15 litres that were previously exempt?

Yes, the provision applies to all hot water systems. However, to reflect the lower risk associated with such a system, the work is not notifiable to the building control body nor is there a requirement for a certificate to be produced that states the work complies with the Building Regulations (unless the associated electrical work is notifiable for the purposes of Part P).