I managed to find a way around this in case anyone else is interested. Here's what I had to do:
- 
Stop new index auto-creation
- Added 
action.auto_create_index: falseto the user override settings for the node 
 - Added 
 - 
Deleted the
metricbeat-7.4.0index (I can afford to lose this data) - 
Created a new
metricbeat-7.4.0-2019-11-06index and assign it the alias 
PUT /metricbeat-7.4.0-2019-11-06
{
    "actions" : [
        { "add" : { "index" : "metricbeat-7.4.0-2019.11.06-000001", "alias" : "metricbeat-7.4.0", "is_write_index" : "true" }
           }
    ]
}
- Re-enable auto-index creation (remove from user config)
 
The index I created quickly rolled over but I'm all good now. None of the metricbeat clients had to be touched to get back to a working state. Hoorah!
Cheers
Dave