OK. So the good news is I sorted it. Side issue - the initial SQL scripts didnt work out of the box - due to database collation with the client I was using. I did a manual override on the first line of the SQL to sort that - but the Payment module was deleted. Then I went to reinstall. That worked too - so no issue.

BUT, the original issue that made me want to uninstall - is to find out where to set the payment gateway credentials. I cannot see that for the life of me. That should be easy :)