hi Rob
you do have a
Galerina there; I strongly suspect that it is
G. paludosa: it is the commonest of several which can occur in
Sphagnum, is generally the earliest of those to appear, has whitish velar remants on the stem (as does yours - well done for taking a shot
in situ, such characters are often removed by careless collecting
)
spores and cheilocystidia look OK as well (though, of course characters such as the nature and extent of spore ornamentation, the type of so-called 'plage' etc, do require oil immersion)
FoS does of course only have a portion of the 60+
Galerina species recorded for GB and Ireland (heaven help people just using
Phillips - which has 4 species plus
mutabilis 
- and one of those is very rare!)
I wonder if the hyphae you have been measuring are not strictly from the pileipellis, but have come from further within the cap? I know I sometimes have difficulties when assessing this part of an agaric's anatomy . . . .
cheers
Chris