"I cannot even download the zip file as it gets halfway though and then says: "Insufficient permissions" so cannot even get to the Etcher stage. Is something strange going on?"
Please ignore. It was an USB problem. I've now downloaded Tinker Board Debian Stretch V2.2.9 which Pi Imager wrote to an SD Card without difficulty. It loads and boots, but after updates is crashing occasionally. I've reverted to an earlier version that seems more stable.