Thanks, but am not sure if that would help - my understanding is that reindexing does not remove the document from the original index, which is ultimately what I want to achieve. Or am I missing something? 
What I've done for now is write a script that time-slices the range I want to operate on hour by hour, and then executes a delete for each hour. However this is far from ideal, as different hours can contain significantly different amount of docs, and I need to take this into account when sleeping between delete requests (as am executing as tasks and don't want to overhelm the system with too many delete tasks. Maybe im over-complicating this! Perhaps a feature enhancement to _delete_by_query in the future to do some of this stuff automatically?
Thanks for the help anyway!