To heat an outbuilding or annexe for those who aren't keen on trusting their life to cheap chinese crap, a mini-split heat pump is a much more sensible option.
With a decent system you should get a real world COP of 3-4, so will get 3-4 kWh of heat per kWh of electricity, so will be paying roughly 10p per kWh of heat. All without the risk of gassing yourself, rigging up dodgy fuel lines, ducting and exhausts and without choking all your neighbours on toxic smoke and soot.
I realise that these things do supposedly separate the heat from the exhaust. But do you really want to trust your life to cheap tinny chinese metal at 100's of degrees C not leaking internally at all, ever?
The big added advantage of a heat pump is that most can reverse and provide cooling/dehumidification in the summer too.
The idea of setting fire to stuff to keep warm is looking a bit caveman-like these days.
One example system, among many...
...not an endorsement, I'd probably spend a bit more for a known brand.