I fixed it in the end… the problem is with Zwift’s CDN (content delivery network). It looks like it’s only partially replicated.
When I tried to download from my ISP #1… it would get about halfway, then be missing a file because it didn’t exist on https://cdn.zwift.com (Innsbruck lamps.GDE)
Then when I tried (twice) to download from my tethered mobile on Vodafone (ouch data charges), it would get further before it would stop again. Clearly the Vodafone network path to CDN.zwift.com takes a different path.
Finally I took my laptop down the street to another neighbour who was using Virgin Active as his ISP… the whole thing came down seamlessly and installed on the laptop, (so clearly using ANOTHER CDN.zwift.com).
Once that was done, I then copied all of the files from laptopc:\program files (x86)\zwift to USB… copied them all across to desktopc:\program files (x86)\zwift, deleted all the partially downloaded files on the desktop and cleared logs… then launched.
so if you’re REALLY stuck… then try using another ISP or else us a VPN to another country to get to it from there.
Please note… there’s some notes on the other threads about ISP compressing/caching etc. and patcher failing because of CRC checks… this is not the case here… the files SIMPLY DO NOT EXIST AT THE DOWNLOAD LOCATION. (I also did extensive tech discussions with my ISP who have strong tech support and root-cause analysis people). Definitely a Zwift problem.
So now it’s working fine and will continue to work fine until zwift do a software update in which case I’m pretty sure it will crater again until I go wandering around houses trying different ISPs.
Final note: Support.zwift.com have still never bothered to answer my direct queries including several log files from different computers which would have instantly showed that the standard answers in the article are not relevant, so they clearly are either understaffed or just don’t give a toss. (@vincent - yes I did try all of those actions, several times including as mentioned in the support queries going as far as reformatting laptop HDD and reinstalling windows from scratch. waste of time that was!)