My plant ID is at best pants but it will have to get better this year.
This plant is growing in the stream running through the sheep field to Berry Leas and looked distinctive enough but I was stumped. Thanks to Steve and James on Birdforum I now know it. Lesser water-parsnip is apparently very similar but this does appear to be Fool's Watercress, aka:
58 Procumbent Marshwort Apium nodiflorum
Canon Powershot SX50