I could only just squeeze my compact camera and enough fingers to operate it, into the hole to get the above image. With the aid of my knife, I also managed to extract a small piece of soil with some of the blue on and using a hand lens, confirmed my suspicion that it was a resupinate fungus.
Running out of time, I had a quick look into other holes and found some more of it inside the entrance of the small burrow of a rodent. This patch was slightly better developed and extended 25 to 30 mm inside the burrow.
Given the tentative nature of my identification, I would obviously welcome some advice. It may be B. atrovirens, but that species is, apparently, only to be found on decaying wood.
I have since found more of this growing in similarly dark places, in Cwmbach.