How to add an internal signal to the waveform viewer in Aldec HDL for a Lattice Machxo3
OK, always seems weird answering your own question, but this might save people a lot of time in the future. 

The big important thing here is that in Design | Settings | Access to Design Objects, you must UNTICK "Limit Read Access to design top level signal only". 

Then, once you have compiled the the testbench you can add all/any signals from the instantiated units that appear in the Structure Tab when simulation is initialised. 

Obv, really. 

Hope this stops other people going insane.