i can't be certain, but it looks like a septic tank, but one that is overground which is unusual for the UK. It is possibly what you said about cow dung... that would seem to be a form of septic tank come generator device...
those little house-box things are fascinating. does the heat travel under the floor in channels like in the Roman days maybe? you're right though, if it is pine you'd have to go outside a lot to put in more.
its not a septic tank!! its meant for stove diesel fuel, the red-colored one. also 12 small doors you saw around that house are not meant for heating or connected to that sphere, but serving solely for basement and/or
the reason theres some ashes is that
when insulating that space, ashes from coal or coke were used in soviet times.
under the floor channels heating made me laugh like crazy :-D
not anymore, alas. the only way is stove
usual firewood is birch sometimes aldertree. pine is rarely used.