OK within a minute of finally posting I solved it. Apologies to anyone wasting tike reading this. 'Duh, duh, Dummy' moment but it seems I am not alone so in case it helps anyone else ...... It wasn't decompressing that was needed but the downloaded ....
I have the same problem, (using M1 Mac on Ventura). Download does not appear to be compressed, system only offers 'compression' and not "uncompress". Download copies onto SD card (formatted in camera) but when trying to update camera just keeps sayin...
