on gravel on the shore well above high water mark
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I still think it is Barbarea not Rorippa, here's why:
The stem leaves were simple
The petals were 5 - 8 mm
The main leaves in the photos are not typical but the leaves at the top of the photo are typical of Barbarea not Rorippa
The fruits were not well developed but looked more square than circular in cross section
Well, the two plant may have been growing together, but the picture here really looks like Rorippa to me. Barbarea has an upright habit and ususally flowers early in the spring (at least in the South). As you say we can't say on the basis of the fruits that are not ripe, but Barbarea has very straight pods (unlike what we see on hte picture). Have you pressed a specimen to run it through a key again?
Thanks for the help.
I have gone back and sat in the rain and confirmed clearly Rorippa
Thanks for keeping us posted
Lat/Lng: 55.927933333333, -4.4773983333333
OS grid ref: NS453733