My dog is itching and scratching day and night, this is been going on now for about 4 weeks and is so bad that she is pulling her hair out and getting quite distressed, iIhave treated her for fleas but she is just the same. She is a 9 year old Yorkshire Terrier.

What did you use for fleas? Shampoos only work for a few days and have to be used for several minutes, intensely. Chemical applications don't all work the first month.
Have you examined her closely? You can see fleas, especially around the anus where they go to drink (or they drink from the eyes).
You really should go to a vet at this point.