OK Ive done my clean install for the second time. The first time I had issues because my user somehow got onto the new partition. Oh yes, OSX 11.2.3 seems to partition the hard drive into two equal amounts which is 2x 1TB in my case, adding the extension .DATA onto the second partition which is not removable with my talents in the disk utility. Im assuming this is for the Rosetta software that my machine doesn't need.
The last time my iMac needed a clean install was when the HD changed from Mac Extended to APFS, I think the new issues were down to formatting the partition. Neither of these issues can surely be done properly without a separate drive running the OS. So a clean install was inevitable.
Shame we don't get warning with thee option to update, like "update 11.2.3 brings compatibility with the new M1 computers and software but is 90% likely to bugger up your system leaving it in need of a clean install" this message would allow me to do it properly when I'm ready. Oh well