Can't find this in Rose and failed to key it out to a sensible answer in Stace III (though I'm very rusty on grasses and may just have gone wrong). Help needed!
No interactions present.
Thanks very much for help so far. I've just added a photo of a single spikelet. There are no bristles arising from the pedicels which rules out Setaria. Lower c.3 florets bisexual, upper sterile. Closest I could guess in the field was Gastridium but that has 1 floret per spikelet. I have collected some.
I've seen Rostraria cristata (in Spain) that is a good match but there may other species. If there is a large population it would be worth collecting a specimen for reference and to get it identifed.
This is a great observation (pending ID):
For grasses that are not common in the UK like this one, there is a free interactive key of the grasses of the World that you can get from Kew's website: http://www.kew.org/data/grasses-db/ident.htm
This observation is still standing as a Setaria on iSpot so could people please switch their agreements to Rostraria, especially the gold-badged botanists? That is assuming you don't still think it's Setaria?
Shows Sheffield Park in Sussex rather than somewhere in London.
Thanks for the update!
Lat/Lng: 51.0, 0.0
OS grid ref: TQ4124