For others looking for a solution here was ours.

Please note: SQL scripts are potentially lethal to the functioning of your database. As with any random code you find on the internet, you should verify it does as described before blindly copying, pasting and running. It is highly recommended that you test (in a test environment) and run a database backup before implementing your solution. Just because this solution worked for us, doesn't mean it will for you.

DELETE FROM ttr_totara_evidence_item WHERE typeid in (1,2);

In our case Type ID 1 and 2 are the default evidence import types for course & certification completions, of which we were happy to delete all instances of.