Thanks for the interest and quick response!
The house is new to me, but is 20 years old, it is a concrete frame that is sturdy enough for the mass, the floor slab is thick anti-seismic concrete. I don't have any real data on its strength, but I'm confident it can hold up.
It's still conceptual, I would like to finish by next winter, so I plan to really refine and define the design before I get building. The mineral blanket you mention, is that glass wool insulation commonly bought in rolls?
And the stove pipe inside the blanket is just thin-walled steel? I had imagined using a 120mm inside a 150mm pipe filling the space between them with the glass wool, but that's only 30mm difference, which would be 15mm thick insulation (stuffed in tight). Maybe not enough. Also, I imagined that it needs thicker walls than normal (.06mm in Spain) stovepipe to not get holes in it at those high temperatures I read about!
I realise that this is going to work only if I get the dimensions correct, I guess I will get the book you mention as I keep hearing about it!
I plan to build it and test it outdoors to see how long the horizontal part can be, and yes the the actual stove part will be central in the house and I'll pass the exhaust through a masonry bench and over to the existing brick chimney which I will line with the 120mm tube.
But the great piece of info you have given me is that the outside of the stove doesn't need to be round, so I can be free to build it myself!