Confidence: I'm as sure as I can be.
Notes: Water Starworts have a reputation (somtimes undeserved) for being tricky, however it is usually necessary to have a plant with ripe fruits to be certain of an ID. Too early for this at the moment I guess. There is a BSBI handbook (no 11 by R.V. Landsdown) on this genus if you're interested - very good and goes a long way to de-mystifying the I issues.