|Small (2mm long) beetle larva found in a Ganoderma bracket fungus|
Fortunately, my first choice turned out to be the right one - the Ciidae, a family of tiny (1-4 mm long) beetles associated with fungi, especially old, decaying brackets and other wood-encrusting forms. They feed on the fungi and several species can sometimes be found in a single fruiting body. As well as being small, the adults are all black or brown, and morphologically very similar - as such they are generally considered to be a 'difficult' group to identify. This is true up to a point as relatively high magnification is needed, x60-x80 or more according to Cooter & Barclay (2006), and there is no easily/cheaply available up-to-date key to the British species - the best is currently Lohse (1967), though there is, I believe, a key currently being worked on. The pair of upturned cerci is typical, and even mature larvae are only weakly sclerotised (there would be more around the head and cerci).
As for which species it is, that's a bit of a guess - I'm not certain the larvae of all species have even been described, but Cis nitidus is a common and widespread species associated with old Ganoderma brackets, so is a likely candidate. Also, given the lack of information on ciid larvae, this is a good opportunity to look at some key features:
|Leg of the ciid larva - the middle left one is indicated by an arrow and bears a pair of tiny claws at the tip (the best focus I could get in the photo, but clearer under the microscope)|
|The posterior of the ciid larva with a pair of brown, upcurved cerci - also note the bristles which presumably have sensory and/or anti-predator functions.|
Lastly, if you do intend to investigate the Ciidae, it's worth remembering that entomologists have argued over the correct name for this family - Ciidae is now widely used, but you might find it called Cisidae, Cissidae or Cioidae...
Cooter, J. & Barclay, M.V.L. (eds.) (2006). A Coleopterist's Handbook (4th ed.). AES, Orpington.
Lohse, G.A. (1967). Familie: Cisidae. In: Freude, H., Harde, K.W. & Lohse, G.A. Die Kafer Mitteleuropas 7, pp. 280-295. Goecke & Evers, Krefeld. [In German]