Big Sur Install Loop

My iMac retina 4k, 21.5inch, late 2015, refuses to install Big Sur. No problem downloading, but the installation process fails after about 40 seconds, reboots, fails, reboots....... I gave up after 10 reboots, but found it looping again today having failed to auto update overnight. I have tried starting from Recovery, reinstalled Catalina, run Diagnostics with no error (ADP000), reset NVRAM, all to no avail. Anybody got any clues about this?