Just heard back from Bisley. They can’t locate a fault. They have fired 100 pellets through it single action without a misfire. They think the problem may lie with my choice of pellets and recommend Bisley Practice. I am sceptical as I tried a variety of pellets, domed and flathead, including Pro-Target Trophy, Milbro Caledonian and Excite Econ, all carefully seated and all with misfires. A mystery, but still very pleased with the helpfulness of Bisley and Umarex.