Further to my posts earlier this week., I couldn't find anything anywhere in the registry about a pending reboot (having looked in all of the usual places such as HKLM\SYSTEM\CurrentControlSet\Control\Session Manager etc).
Safe Mode install also rendered the pending reboot message.
Having tried no end of combinations of restart, sfc /scannow and the System Update Readiness Tool I resigned myself to the fact that I wasn't going to find a fix. I'll wait to see if Microsoft come up with one but I'm not holding my breath.
As a last resort I've tested performing an Upgrade install of Windows from Media with SP1 already integrated and that works. SP1 reports as correctly installed and everything seems right with the world. It takes absolute AGES though so be prepared for a long wait. Also reported that .Net 4 needed to be repaired following the install but it took care of this automatically and, another reboot later, everything seems to be back to normal. The one major problem with this method is that it wants a new Product Key to be entered so this could cause you further problems and I'm not sure what the licensing implications are there.
If anyone is reading this as they have th c34 error, MS have a fix now that doesn't break things. You can find the step by step instructions here.