Many thanks Adrian for the tip!
I had not considered using FTP>SSH>Linux so checked it out myself today.
Saying that Windows users can skip the first two steps with Cygwin, a collection of open source command line tools similar to Linux.
You just need to download it, remember to install the zip and unzip packages (they are not default modules) and set your path variables; the installer guides you through that.
I ran a test on the most recent monthly file, Accounts_Monthly_Data-October2020.zip. For the record, here are my checksums for that file: -
Size: 1876773957 bytes (1789 MiB)
Of all the files processed in my regular Zip software, there were 227,536 errors out of 233,107 files, leaving only 5,571 to work with.
I then processed these same files in Cygwin using the following commands: -
zip -FF Accounts_Monthly_Data-October2020.zip --out recovered_Accounts_Monthly_Data-October2020.zip
There is also an '-F' switch which can be utilised for zip files with minor damage, apparently.
This returned 231,182 files, a considerable improvement but with 1,925 short. Unzipping these files in turn with the command returned 229,759 files from the recovered zip, a further shortfall of 1,423.
I am still investigating the reasons for the shortfall on the recovered file, but apparently others have tried rerunning this command to create a second recovered zip file and extracting from that!
The clear message is that this is a considerable improvement but it is still not 100% perfect. Perhaps mitigation by downloading both the daily and monthly files and cross-referencing them against those files already processed is the way to go, I don't know - that would depend on the use case.
And if anyone knows of any 'better' Linux commands to rival Cygwin unzip (or other installations for that matter), I would be interested to hear about them.