You may need a solicitor. Have a look at your deeds to the house searchlight, you should have a legal access to the back of your house which enables you/your gardener/window cleaner or other neighbours to walk around whenever you want to, no permission required. The land for your house will delineated in red on the plan and the access (the width usually measured) and probably only on foot - no vehicles, will be mentioned in the house deeds. If he is blocking the access a letter from your solicitor should work. Otherwise the solictor could advise you. Do any other neighbours have an access?