Thanks for the replies.
Creating a new index was really what I was getting at with my last comment but I hadn't appreciated that you could effectively do a 'reindex by query' which I have now found in the docs.
I have started a reindex that excludes the superfluous documents and that seems to be working and should complete within 2-3 hours at current rate. Thanks.
How do I switch my clients from the old index (e.g. 'foobar') to the new one ('e.g foobar-lite') given they are actively writing to 'foobar' via the RESTFUL API. I have experimented with a local docker installation of ELK and it looks like creating an alias such that 'foobar' refers to 'foobar-lite' does what I want (of course this requires that I delete the old index first as it won't allow an alias with the same name as a physical index which will be nervous moment on the real server)
Is this a reasonable strategy?