Thank you for your question.
The York Midi defra stove needs a free space of 100mm minimum above it in order to heat the room efficiently. Any less than this and the stove won't be able to heat your room properly.
We normally recommend a space of at least 150mm around any part of the stove.
Ideally you would need a stove of 500mm height or less, here is a list of suitable Defra stoves 500mm and less - https://www.directstoves.com/stoves/defra-stoves.html?overall_height=451mm-to-500mm
If none of these are suitable, why not ask your Hetas installer about making the fireplace opening larger? - https://www.hetas.co.uk/find-installer/
Hope this helps?