I chanced across a patch of Porella arboris-vitae during a site inspection at Old Castle Down this afternoon. What a super looking liverwort. I wasn't really looking for bryos but the dark patch on a pale limestone background (see lower photo) attracted attention immediately.
The funny thing is that a year ago I did some habitat surveys (including quadrats) about 20m away, and we even had lunch in the grassy area behind the rock in the photo. I didn't spot it then as the patch is on the shady side of the rock facing the woodland edge. I only saw it today as I made a quick inspection of oak buds for Purple Hairstreak eggs.
There didn't seem to be any doubt about the ID but I took a small sample home and nibbled a tiny fragment. This was about an hour ago and the tip of my tongue is still numb - it makes Water-pepper seem quite mild in comparison!
Looking at the Atlas, the nearest Welsh population would appear to be about 50km away. There are closer populations on Exmoor though (just like the High Brown Fritillary!).