Well it seems Bethesda decided to stop hosting the latest official patch as a download. I hope this is temporary.

I read on tomlongs site that the patch included with SI does not include the fixes for the Oblivion Launcher or the Textures BSA.

Could I maybe get around this problem by first installing the 1.1 patch included on the Knights of the Nine retail DvD, then install the Shivering Isles Retail patch?

It took me half an hour to find it, but I think this page looks pretty promising. That is, if the file links there are still working.
http://support.gamesas.com/patches.php

I must admit, they really went out of their way to make the patches unaccessible this time.

Many thanks those links seem to work.