domain mapping problem after WP3.6 and MD1.2.3 update

I have just updated to 3.6, and installed multi domains 1.2.3. Now my domain mapping isn't working - when I type in the mapped address I am taken to the page to register a site.

Any tips on what to start with in sorting this out?

Network is at rurally-hosted.co.uk (but it's not pretty)