get in touch with an agency (google dental nurse agencies)and they might be able to get you some temp work in practices maybe?? if you want to work in hospital you will usually need to register with nhs jobs and they send you alerts when jobs become avalable so you can apply over the internet which is really quick and easy
if you want to do saturday work speak to the dentist you work for and he may know a collegue who needs an experienced nurse to help out?? as ekw says you can speak to the matron of the hospital and she may let you observe on some clinics!?!