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I bought this post to the attention of Peter Chandler. this is his reply to me:

"Two species of Mycetophila known to develop in this fungus, M. laeta and M.attonsa, neither found in Britain, but attonsa has p-d setae on hind tibia and laeta apparently differs in body coloration so can't be sure what it is. The fungus is said to occur in Scotland so I have asked some people there to look out for it."