Allright so the datapump is a red herring.

The problem is with SchemaCacheDir.

I had switched it on, and was seeing information generated, but there
is obviously something I am missing. If I switch it off the problem
goes away.

I was just trying to save a few nanoseconds on opening the network
connection, since it is only 30to40kbs. So in theory if I keep local
copies of Schema I will be faster. What have I missed?