Clare:

Are we talking about SQL2008 or SQL2008 R2? The location of the setup file that rebuilds the system databases is different for R2, and the procedure is not well documented.

I went through this exercise last week - fortunately for me it was a non-prod system I deliberately broke so I could learn exactly how to fix it if the Master DB was actually lost.

For detailed steps see my second message in http://www.sqlservercentral.com/Forums/Topic1437039-1550-1.aspx