Photos appears to be stuck on downloading originals

27" iMac Mid 2010 running High Sierra 10.13.4

I recently had to do a clean install of High Sierra, so essentially a new iMac. Now trying to download approx 26,000 photos from my iCloud Photo library. I want to keep all my original photos on my iMac. Photos has been stuck on "Downloading 23,544 Originals" for days, the number doesn't not change. The Mac is connected to my router via ethernet cable and machine has been left awake.

There is plenty of space on my hard drive 913GB for 30GB worth of photos. There is plenty of storage on my iCloud storage plan. I have left the iMac running for days now and still no progress. I can see all of my photos thumbnails , some are full resolution, others have the little clock bottom right when I click it open, but the little clock only shows a quarter full and then stops. This seems quite random, some photos from 2014 showing up as full res, others in the same album not fully downloaded.

I have "Download Originals to this Mac" selected in Photos Preferences iCloud. If I try to turn off iCloud Photo Library in Photos Preferences, I get the message 'Your iCloud Photo Library is currently updating. If you turn off iCloud Photo Library, photos and videos that have not been fully downloaded will be removed from the Mac etc etc....

I have iCloud Photo Library switched on on my iPad Pro and my iPhone 7+, both with "Optimise" storage selected.If I click on a photo on either device it pops up full resolution almost immediately, the little clock flies around. On the iMac, same photo stuck on a quarter turn of the clock.

This is very frustrating. I have a Time Machine back up that doesn't seem to want to work either, I can see all my files but 'Restore' is greyed out so can't even restore my Photo library that way.

I can't believe the download should take this long, especially as the progress bar in Photos isn't moving at all.

Any suggestions, as to how to resolve this would be much appreciated.