There are a growing number of CASTEP questions and answers on the CASTEP forum at JISC. You should browse the archives to see if your question has already been answered before, and if not, then ask it there. Many members of the CASTEP user community as well as all the CDG will then see your message and be able to respond.
CCPforge Download site for UK academic users, as well as bug tracking.
The United Kingdom Car-Parrinello Consortium (UKCP) is a group of researchers across the UK who develop `first principles' quantum mechanical techniques for studying atomistic systems and apply them to a very wide variety of systems. Historically, CASTEP was the flagship code for the UKCP Consortium, and much of the CASTEP development work is still funded by this route. Members of UKCP have pioneered many innovations in algorithms and applications, and all of the CDG are current members of UKCP.