Most dental nurses are covered by their employers indemnity insurance as part of the practice. However this usually only covers the nurse for claims etc and doesn't include legal advice or representation at hearings by the GDC.
The GDC standard document states that if YOU are carrying out tasks within your scope of practice then YOU must make sure that you are properly trained, competent and indemnified. SO...if something goes wrong and you weren't properly trained then you would be responsible. However if the dentist was aware of you doing this then they would also hold some of the blame.
If you're currently training and performing your impressions and radiographs as part of a training plan, and something goes wrong then you should be fine if you are supervised by a trainer.
Hope that answers your questions