We have a test environment with a virtualised DC in it and an external DC as well. Just trying to get a definitive answer as to whether if the external DC is down, will the cluster fire up so we can start the virtualised DC? 

Most of the net says no, the cluster won't start, but that is for 2008R2 and before clusters. 2012+ seems different from some forum posts and doesn't even seem to require quorum to be reachable to start the cluster.