So having fiddled with the migration via drush a little, and having restored the database a few times along the way (thanks drush ard/arr) I now have all my content in D8. Seems like taxonomy terms didn't make it (but vocabularies did).

To get the migration to complete without skipping all the nodes completely I had to apply a patch from Skip the row when we don't recognize the the field type to get nodes working.

Seems like Taxonony terms also need work.

And the interesting thing about the nodes that did migrate correctly is that they show up as blank on teaser or full view because the 'Filtered HTML' input filter has a broken component. In the description of the filter when editing a node (where you can see the content that was migrated) the broken component says: 'Missing filter. All text is removed'. Handy! If in doubt let's strip it out! So another manual change would be to fix the newly imported 'Filtered HTML' input filter once a migration is complete.