You may have to be a bit of a detective, but there are often many potential leads. Look at old maps in your local library. They may also have reference material on the garden, especially if attached to a house of significance. Try the local historical society, or county Gardens Trust (see http://www.gardenstrusts.co.uk/contacts.htm): they may already have undertaken some research, or be willing to help.
See the links section for national organisations that can help, and publications that will have general information but could also have specific detail of relevance. We may also have information: contact firstname.lastname@example.org And don’t forget, ask around: local knowledge is often the most valuable.