Tip received from our ops provider:
sed -i 's#archive.ubuntu.com/ubuntu#ftp.heanet.ie/pub/ubuntu#g' /etc/apt/sources.list
sed -i 's#security.ubuntu.com/ubuntu#ftp.heanet.ie/pub/ubuntu#g' /etc/apt/sources.list
And then swap the target to the closest/fastest mirror at https://launchpad.net/ubuntu/+archivemirrors.
I’m trying this now with a US mirror (Enzu) as being most suitable for CircleCI, it is working fine even when building in Docker locally (in the UK).