After some experimentation the https update seems to be the glitch. When I save a https page from the site and load it in my archive that preserved page isn't calling up the page background or some style sheet material, which results in a default white background and, for some reason, centralised text throughout.

I suspect this is a feature of the ssl callup objecting to being hauled to a different url but I don't know enough about it to diagnose properly.

The workaround seems to be to delete the s after the http and save the resultant non-https version, which archives just fine.