We do advise that a survey should be carried out before a stove is chosen to inspect the condition of the flue system itself, to check for mortar deficiency, bends in the flue system etc this should be carried out by a qualified HETAS engineer to conclude that it will work both mechanically and thermodynamically. Once this has been clarified by the surveyor/installer, dimensions of the existing & possible opening size after any remedial work will be taken so that we can then assist you with choosing the correct appliance.
You certainly should not have smoke bellowing into the room, At this point we would recommend you should not use the appliance until a HETAS engineer has inspected your appliance. With regards to you being advised by a builder, I’m afraid a builder shouldn’t advise in these situation as this is a controlled environment and a HETAS engineer is qualified and are aware of the requirements, and adhere to ‘Document J’ of building regulations. A spinning cowl is manufactured for creating a draw in most cases if the appliance was correctly installed you should not need to purchase an additional spinning cowl.
A lot of people have the impression that if you measure the room itself then you can work out the required kilowatt output that will be required. There are many things to consider when analysing what kilowatt is required for example; is the ceiling high, do you have cavity walls, do you have a lot of glass in the room maybe french doors leading to a garden. This is where heat can be lost as the glass dissipates the heat dramatically, What temperature do you on average, have your thermostat set to at home? All of this information can be used to advise you as to the kilowatt output requirement.