Confidence: I'm as sure as I can be.
Notes: This looks rather interesting, and it seems to have unusually long fruits. There are a number of recognised segregates of the P. aviculare group, but their taxonomic treatment has varied and there is much disagreement. In recent times, most inland plants have been called one of: P. aviculare, P. arenastrum, P. rurivagum (uncommon or rare and misunderstood), P. boreale. What you show here is none of these. There is an unpublished account by Peter Sell that will expand this group considerably, and rightly so in my view. During work on coastal plants I have come to know some of these, but this is not one I recognise (well, I might guess, but I won't). For the moment, just call it a knotgrass in the aggregate sense. I think the knotgrasses will be in the next volume of the 'Flora of Great Britain and Ireland' (5 volumes when complete, expensive!). I have seen the draft account and am impressed with it, but there will still be problems. Alan.