We've found this a couple of times recently, not too far apart near Haweswater, on ash and oak. In both cases the 3-septate spores had hyaline cells at both ends. The sample I looked at on ash had spores 19.2-20.8x5.6-8μm, while the oak was 27.2-34.3 x 8-9.6μm.
On the Cumbria Fungi Group Paul Nichol definitely agreed with the second one and suggested H. pulicare for the first. His source indicates spore length of 20 - 30 microns, while the account at http://fungi.myspecies.info/all-fungi/h ... m-pulicare says 26-37 x 7-12 µm. There is an overlap with H. angustatum but I think the hyaline cells may be diagnostic.