Hi Tony,
this a very interesting find, likely new to UK or even Europe, but I did not check on the Internet. Indeed ascospore morphology is disctinctive and readily points toward the genus Spirodecospora.
It might fit well S. bambusae described in 1998 on Bambusa from Hong Kong, but a more recent study (2022)introduced new species, including S. melnikii mainly differring from S. bambusa by DNA differences and Sasa as known host.
I attach the link to the paper so that you can compare your fungus to those dealt with in this study.
Good luck!
Jacques

https://doi.org/10.3114/fuse.2022.10.09

Hello Jacques, Many thanks for your instant recognition of the genus. You are right in that no species of Spirodecospora seems to have been recorded in the UK, perhaps Europe also as you stated. I looked clsoely at the paper you referred me to (Sugita et al, etc.,) and overall the measurements I have correlate more with S. melnikii than S. bambusicola. Just! As far as accepting the genus I think this is for certain, as described in the paper virtually everything fits. I did note that the authors question if these are germ slits at all, and could be something esle, but the spores easily fit with results I have here. There is no molecular data available for S. bambusicola and no sexual morph information for any species, yet I might have access to both with this sample. The substrate is of the Bambuseae family so suggests S. bambusicola if we follow the authors preference, but I feel this is a slim fit and S. melnikii could have alternatives in Europe. The sample I have has hundred or so of available ascomata and by the looks of things, still maturing. I hope to have access to DNA extraction through Aberystwyth University in Wales. I will try this direction to see if we can match S. meinikii, if not, it probably is S. bambusicola. I can always send you a section if you were interested. 
Many thanks for your guidance.   Tony

Hi Tony,
be aware you are about to become the European specialist of Spirodecospora!
especially if you obtain sequences from your material.
I gratefully accept your offer to share a small part of your collection. I am eager to see these spores and their mysterious germ slits.
Gently air-dried and wrapped in paper, no plastic nor metal.
My postal address is
"Las Muros"
F. 09420 Rimont
Thanks a lot!
Jacques

Will do. Thks again   Tony

Hello Hi Tony and Jacques,
If it is viable I wouldn't mind culturing it so I can check if it produces asexual morph in culture.
Regards.
Angel

Hello Angel,

Yes, more than happuy to send a section to you. Problem is one has gone for DNA extraction and just sent Jacques another. I have asked for the main section to be returned from the Laboratory which they have agreed to do, and will send this to you as soon as possible. This could take a few weeks. If that is OK, then please advise your address my email is: tonychat47@btinternet.com

regards  Tony

Hi Tony and Angel,
I don't know how long the spores will remain viable.
I think it would save time if I forward to Angel the specimen I receive as son as I have studied it.
So Tony can keep the sequcened material.
Jacques