Yeah figured out what is wrong. Fixing it is however challenging for a few reasons, mostly because Elastic Search's idea of backwards compatibility is to laugh in your face.Nop.
When es essentials is disabled, normal search working
When ES essentials is enabled, autocomplete working, but if press enter to show search for query, this error above shows
That shouldn't be required, it is just the query that needs fixing. Which I've mostly done except there is a few niggling bits which are leaving me baffled atm.P.S. Just tell me will I need to reindex search again after fix