Xylariaceae family

Fruiting body with raised bumps (left), dissected mushroom showing concentric rings (right)

Daldinia concentrica or (Carbon Balls, King Alfred's Cake, cramp balls, and coal fungus) 

According to legend, King Alfred once hid out in a countryside homestead during war, and was put in charge of removing baking from the oven when it was done. He fell asleep and the cakes burned. Daldinia concentrica is said to resemble a cake left to this fate.

Confirmed by the presence of concentric rings when sliced open.  Also, a charcoal-like residue left left when rubbed between fingers.   First found at Newport Forest in 2000.

The fungus is a useful form of tinder for fire-lighting. The brown variety is usually too heavy and dense to be much good; the black variety is lighter and better. It does need to be completely dry, whereupon it will easily take a spark from a firesteel. It burns slowly, much like a charcoal briquette, with a particularly pungent smoke. Once lit it is quite difficult to extinguish, but fragments can be broken off and transferred to a tinder ball to create an open flame.