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Lolium perenne has (usually) awnless, sessile, spikelets, whilst this has awned, stalked, spikelets. I think that it may be Bromus, but it is not the common Bromus hordeaceus.
Thanks all for helping me to identify this grass. Can you please, if you have time, tell me how it differs from Bromus Hordeaceus? I've had a look at other pictures/descriptions and they look v similar. I don't have sufficient knowledge/experience to see the difference myself. Thanks again.
... are that the inflorescences are too lax, that is the spikelets aren't clustered enough. Beyond that I'm in the same boat as you - the different Bromus species all look very similar. (And I've never knowingly seen any Bromus - as opposed to Anisantha, Bromopsis and Ceratochloa - species other than Bromus hordeaceus.)
You may well also have a different habitat - in my experience Bromus hordeaceus is mostly a grass of hedgerow margins.
Lat/Lng: 51.8, 0.9461
OS grid ref: TM032153
On the path along the bank of the river Colne (tidal), between Rowhedge and Hythe Lagoons.