Linux repos

Answering my own question:-

Sort out your own /etc/apt/sources.list file

sudo apt-get install apt-mirror

edit /etc/apt/mirror.list to define the repos you want to download (from your assumed working /etc/apt/sources.list file)

sudo apt-mirror

I'm starting to worry that Netgear will drift even further away from their current level of support for poor Readynas users.  Admittedly that support is near zero, but taking servers offline could happen.