Rob
You have done a good job with trying to give all the necessary information. A view of the gills showing the number and attachment would be good
and was there a recognisable smell?
The fruit-body and location has the look of Hiemalis but the Cheilocystidia rule that out.
Another possibility is M erubescens a species that bleeds a milky liquid and your photo shows some signs of this on the stipe. Unfortunately the spores and basidia rule that one out
Mirata is the right shape, the right location, the cheilocystidia is OK, the spores good, the stem hyphae good, the number of lamellae not sure but the only fly in the ointment is the basidia.(Could you loose that picture
) I know some species (galericulata) start the year with two spored basidia and end with four but I have no note of mirata being a species that does this.
Mirata is by far the best fit I can see so unless there's a Mycena man out there with a better suggestion I would go for that.
Mal