I've just installed Joomla! 2.5.1 (via update from Joomla 1.7.3) and installed the latest version of K2. (getk2.org/downloads/?f=K2_v2.5.4.zip) However, there is no menu item for K2 in the component dropdown in the admin section, and there is no dashboard that looks like it is meant to appear underneath the default Joomla dashboard. The extension manager says that the K2 component is installed, as well as all the modules and plugins from the install package. But I have no way of controlling anything in K2 through the component...

Any help would be appreciated,

Simon

I just installed K2 the latest version on joomla 2.5.2 it works fine.
Have you tried to create a new install on a test site with joomla 2.5.2 and K2.
You may have issues with add-ons that are designed for the older version of joomla