I found another thread Data table :hide column Count that confirms that the Metric column cannot be removed / hidden so I used a bit of a hack to get as close to the desired results as I could short of writing a plugin.
I added a dummy numeric field to my document that has a null value for all records and in the data table I select the Metric as Max (or Min) of this field and use a label of _ or . to make the column as close to invisible as I can. Sadly column label doesn't accept markup so doesn't work but this is what I'll have to work with for now.
Definitely a fundamental building block viz that is missing from Kibana.