Thanks Matias and Winrunner for replying. And apologies for the dilatory response.
Just a clarification: I am receiving 42 errors - not warnings.
I first checked properties in InstallationDir\CAL\Desktop\Composite.Presentation.Tests\ and the child directories and Read Only is unset. So, this was not the cause.
I then checked to ensure I had Team Developer and Tester Tools - and I do indeed have Performance Tools, Code Analysis Tools and Testing Tools installed.
Finally, I tried unloading the following six projects and now the solution compiles.
While this is more of a band-aid than anything else I can at least run the solution. If anyone has any wisdom on why this occurs even though I have Testing Tools installed I would greatly appreciate hearing it.
All the best,