Software Update Error

So this morning I opened iTunes and was invited to update the software to the new iTunes 11.

There were also several other programs with updates. I clicked continue and proceeded to download 1.8gb of updates.

At the end of the lengthy download, I clicked restart (as required), but just before restarting, a window popped up displaying:

"Some of the items cannot be installed, an error occured".

The computer restarted but my iTunes hasn't updated to 11, it's still at 10.

I tried a software update again, but this time it only downloaded 900kb worth of some quicktime stuff. Upon restarting I get the same error message and no iTunes 11.

How do I get my updates?!

Macbook Pro late 2011, with OS X Lion.