Just had one of these installed Esse 301
It is small at 5kW but it is mainly for chilly evenings and for extra comfort in the deths of winter and importantly it sits straight into the existing fire place - we have a victorian wooden surround and tiles that we didn't want disturbed.
It is connected into the existing chimney - after it was swept and tested. No extra work to the chimney if it passes the test. Traditional fireplace needed throat removed and backplate fitted.
Cost about £1k for the fire and £1k to have fitted - which included a new slate hearth cut around the wooden surround.
Rather more expensive than I had intended. Finished Friday and not yet commissioned - so it had better be worth it!
There is a smaller one at 3kW but I think it only takes coal.