Thanks for the response. Yes I read the KB yesterday, the registered host entries appear to be across hosts, host_attributes, host_memos tables with relating answer files and licenses in the cache_item and cache_item_file tables.
So unfortunately it's not a single delete statement, I'll work my way through them. A "forget hosts" action would be a good addition to the fling gui/next version, if any of the devs are reading!
This is because AutoD does not apply the host profile post deployment if the host is already "recognised", so it would be good to have a method to easily clean them out without having to reset the db and redo all the rules.