Growing on limestone grassland, often on ant hills.
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you have put it in fungi instead of plants,
Thanks for spotting that one. My edit seems to have created a second id. Oh well.
Seems to key out (in Rose) to Galium sterneri (Limestone Bedstraw), except that you say it isn't mat-forming. Maybe the mat comes later! Or it could be Galium pumilum, Slender Bedstraw, but that is apparently very rare and has cream-coloured flowers and the photo looks much more like white than cream. But I expect you've already found all that out yourself! As you say, you probably need the fruits. It was useful to look at a non-marshy bedstraw for a change for me.
Anyway, it isn't a potentially poisonous fungus!
I have gone through the key in Stace (ignoring branches that need fruits but direct me to things like cleavers) and it agrees with Limestone Bedstraw. However, I will wait a bit to see if anyone knows if the mat forming bit is variable.
Lat/Lng: 54.23803, -2.86074
OS grid ref: SD439828
Woodland and short grassland over limestone. Cumbria Wildlife nature reserve. Stream in valley bottom with heavy, damp soil.