iMac 2017 Catalina to big sur constant reboot

am trying to upgrade to Big Sur from Catalina (10.15.7) on a 2017 iMac 27" 5K

first tried to install in late January - i think i downloaded v11.1 - but was having the restart issue mentioned in the forums here, couldn't get it working so after a few hours finally gave up and using option on restart rebooted back into Catalina and then deleted the "install big sur" application from Applications folder. I tried resetting nvram, smc, ran disk utility and fixed disks, unplugged all peripherals etc

Yesterday i thought I would try again - this time downloaded 11.2.3 and tried to install but same problem - progress bar gets to about 20% and then just keeps rebooting (about every 5 minutes).

Before I erase the drive and try a completely fresh install is there anything else I can try? I have seen several posts here and elsewhere with people asking the same question but have not found any explantation of why this may be happening.