Apethymus filiformis
Some online references apply the common name Common Sawfly to this species but this doesn't seem to be widely recognised and used. The species may be common but there don't appear to be many records of it compared with lots of other sawfly species, so it doesn't seem to be a very appropriate name.
Thanks to Andy M for identifying this sawfly larva.
Apethymus filiformis, North Elmham Cathedral Meadows (Norfolk, UK), 25th May 2019