Hi Graham, and welcome to the forum!
smxi is a third-party script and may or may not fully support your system (I think you already found out about that). You will rarely need anything beyond apt to upgrade your siduction system. There's a paragraph or two in the manual on this topic, that you really should review.
With your sources set correctly, all you should need to do is
apt-get update
apt-get dist-upgrade
apt-get clean
and occasionally, if you are prompted,
apt-get autoremove
Also, keep an eye on the Upgrade Warnings forum when large transitions in Debian can cause some excitement.