A quick google shows that wget has a --cache option which can be set to "off" to bypass upstream proxies.

Yes, the "--no-cache" option *does* allow me to wget the current (correct) version of the file.
Not only that, but it apparently also clears (or updates) the upstream cache, and the "--no-cache" option becomes no longer needed.
Furthermore, having run "wget --no-cache ..." on the Ubuntu machine, the other machines immediately started downloading the correct file.
I consider that to be proof that the cache was definitely upstream of me.

I still don't know precisely where that cache is, but I've sent off an email to my ISP where I accuse them of sending me outdated files that I don't want (and still charging me for the download).
No response from the turds, yet.

I thought I had sent a post about this last night ... but I don't see it now. Presumably I didn't get around to hitting "send".
I apologise for that.

Thanks to all who responded !!

Cheers,
Rob