You should always take with you an appropriate map. This is most often going to be an Ordnance Survey or Harvey’s map at 1:50,000, 1:40,000 or 1:25,000 scale. You should also have a magnetic compass, such as a Silva Expedition 4. It goes without saying that you need to know how this equipment. In addition, you could take a GPS unit or an app on a smartphone, but these supplement rather than replace a map and compass.