I suppose the very short spore bearing stage compared to the lifespan of the fungus goes a long way to explaining its rarity.
1 in every 1000 Birch being infected on average springs to mind and even this seems very high for the UK.
Yes, I think you are right. I've read everything from 1/1000 to 1/15000, and it takes up to 20 years from the tree is infected til it is dead and the spore bearing stage develops. Very often the tree is felled or dies of other things before the fungus reaches its reproduction stage.
And I guess there are great regional differences in how common it is. Where I live now, just north of Oslo it can be found quite often, but Where I grew up in the woods and mountains north-aest from here it is very rare.
I more or less stumbeled into this old birch on a hike earlier this summer in the area where Igrew up. There was only one little twig with leaves on it, the rest was dead, and it actually hadd two conkers on it. Maybe the chaga will reach it's spore bearing phase on this one in a couple of years.
Concker number one.
And number two.
The conckers wasn't to big, but I got two nice pieces of it (I didn't realize that that ant was crawling on my thumb before I saw the pictures).