No interactions present.
... the petals and sepals are roughly the same length; in Rorippa sylvestris the petals are about twice the length of the sepals.
There are also Rorippa islandica and several hybrids. You can probably ignore them as being absent from your area.
Thanks for your helpful comments. One of the reasons I was wary of plumping for this id is that in my reference book, it says it's common 'throughout, except in the north'! Ah well...
... it not being a rare or local plant.
It turns out that in this case the north is north of the Highland Boundary Fault.
You (and I) are in the centre of its distribution - a triangle with one corner at Birmingham stretching north to cover south Lancaster and south west Yorkshire. It's thin on the ground between there and the Scottish Central Lowlands, but that may be due to a lack of observers, rather than any actual scarcity - there is dense cover of old records for the Fylde and Lancaster area, Furness and south Westmorland, and the Vale of Eden, which look as if they are due to old surveys for (vice-)county Floras.
I've found a couple of likely occurrences (I've still to have a good look at the photographs) in the last couple of days, one in a ridiculous habitat - dry ground outside the walls of a public library.
Thanks so much. I always find your comments helpful and interesting. Makes me realise how little I know!
Lat/Lng: 53.609, -2.30277
OS grid ref: SD800125