Small Fire, Warm Heart
I find that no matter how big the fire, my back is always cold and infact its the sight of the flames dancing which offers the most warmth. Provided you protect the grass (raise on bricks etc...) you are able to feed it with small twigs and pieces of wood (collected on your afternoon walk) in order to have a small controlled fire.
If there ever was a problem, it would only require a small amount of water to extinguish or failing that it is small enough to stamp out.
Gelert Solid Fuel Stove & Fuel £3.50 inc. Fuel
Indian builds small fire - Keeps warm
Whiteman builds big fire, Keeps warm collecting firewood