From the Builds page; http://www.piriform.com/ccleaner/builds
clicking the Download button for the Portable build briefly takes you to the right page before automatically going back to the Piriform home page.
Manually going to http://www.piriform.com/ccleaner/download/portable does the same, hitting the Back button takes you back (and it stays there) to the Portable download page, but again, hitting the Start Download link takes you back to the Home Page.
Seem to recall this happening the last time a new release was done.