Not the fungus, which is Ascocoryne sarcoides, but the insects around it.
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Mark_at_Magdalen, I think you're right. Thanks. It'a quite common to see other springtails, more elongated and darker, swarming over moist deadwood and fungi. I wasn't familiar with this type.
Lat/Lng: 51.4093, 0.0865
OS grid ref: TQ452698