The aim of the study is to find out if the experimental medicine, YF476, can make gastric carcinoids, a rare type of stomach tumour, shrink and disappear. Gastric carcinoids occur mainly in patients with chronic atrophic gastritis (CAG), a condition in which the acid-producing cells in the lining of the stomach can't make acid. Acid production is controlled by gastrin, a hormone (chemical messenger) that's released into the bloodstream. If the stomach can't make acid, blood levels of gastrin rise. High blood levels of gastrin in patients with CAG can cause other cells (ECL cells) in the lining of the stomach to grow and, over the years, to give rise to gastric carcinoids. Gastric carcinoids are usually benign, but they can become malignant. Therefore, patients with CAG and gastric carcinoids have the inside of their stomach checked regularly, by gastroscopy, to see if the gastric carcinoids need removing surgically. A gastroscope is a thin (1 cm), flexible tube at end of which is a mini video camera, which enables the user to inspect the lining of the stomach and a 'snare' to take samples of tissue (biopsies). YF476 (netazepide) is a gastrin receptor antagonist (blocks the effects of gastrin), so it's a potential new medical treatment for gastric carcinoids in patients with CAG. Up to 10 of these patients will take YF476 daily for up to 12 weeks. If they benefit from that treatment, they may take YF476 daily for up to another 52 weeks. They'll make several outpatient visits for tests, including checks on the safety of YF476. At some of the visits, they'll have a gastroscopy. At each gastroscopy, the gastric carcinoids will be measured and biopsies taken for laboratory tests.