Goldspoon answered most of it, any luck with HETAS guy yet?8m x 4.5m x 2.3m just a square room no recesses.
Will need as least 5.9kW stove depending on how well the room is insulated (8m x 4.5m x 2.3m ÷ 14 = 5.91) 1kW per 14 cubic metre
Be careful when buying the stove, check for the minimum distance from the back of the stove to the non-combustible wall on the stove manufacter plate which they don't tell you if buying stove online, my stove is 60cm!