I have had a sore throat for a long time, but I also have quite bad hay fever, could my allergies be a cause of my sore throat?

Absolutely. "hay fever" often causes sneezing, runny nose, and nasal congestion. Which can lead to dripping at the back of the nose. Which goes down the throat. This could definitely cause a sore throat...See an allergist. There are many good treatments to help you feel better.
M.D. visit. A scratchy throat may b associated with allergies. Since it has been prolonged though an examination at your physician's office should be done. If you have other symptoms such as fever, night sweats or easy bruising this will be important information for the visit.