Thanks for that, Nutcracker.
Of the three planthopper species of interest, all I know is that Stenocranus major (which seems like the best bet, all things considered) has been linked to Phalaris arundinacea - at least according to British Bugs. I did a Google image search when I first read about this, but it didn't seem to bear much resemblance to the grass in my photos. However, I had another look at it this morning, and some of the images included on the Nature Spot site look similar to me:
https://www.naturespot.org.uk/species/reed-canary-grass
But then again, so do the meadow-grasses you suggested!
No doubt it can be found on a variety of different plant species, and so it's probably not worth pursuing this line any further.
For info, I've submitted the sighting to iRecord as a probable Stenocranus major, and I'll see what they (eventually!) come back with.
Regards
Paul K