I had two DCs on my network but one has died and, for various reasons, won't be coming back online. I have used AD Users and Computers to seize some of the roles but when I run a DCDIAG on the remaining DC it tells me there are roles still held on the missing DC. How do I seize these?

This Microsoft KB article explains how to use ntdsutil from the command line:

Using Ntdsutil.exe to transfer or seize FSMO roles to a domain controller

It will attempt a safe transfer first, and then seize the roles if it can't locate the current role holder.

Dave

Thanks Dave, I have seized all roles now. However, the remaining DC doesn't appear to be functioning as a PDC. DCDIAG reports the following:



Do I just need to wait for it to sort itself out? Or should it assume that role straight away?

You may also need to clean up data left in AD from the dead DC. Have a look at this article:

Clean Up Server Metadata