Sorry to answer myself, but it seems there is a gap in the documentation.
All I had to do was start the Mongodb server, and suddenly I see the Tyk dashboard appearing.
There were no logs etc indicating any issue connecting to MongoDB.
There are various flavours of Mongo for Ubuntu - the one your instructions installs does not start anything by default. I think this was changed quite a while ago to reduce the unsecured default installs, and also due to introduction of the dreaded systemd on Ubuntu.