Here is the GDC's line on the matter from their standards document:
Standard 6.2.2 says: You should work with another appropriately trained member of the dental team at all times when treating patients in a dental setting.
The word "should" means that its not absolute. However ideally you should be working with another nurse at all times as they need to act as a chaperone for the patient, mix your materials, be there in case of a medical emergency etc. It should be no different from when a dentist is taking impressions. That being said, if a nurse isn't readily available you should adopt an open door rule whereby you leave the door open so you can shout if there is an emergency or threatening situation then other members of the team can hear you.
