2013 photos & vids here
eff37 on Flickr
So, I'm revisiting this as I've found them again in almost exactly the same place... The plant is Yellow Iris.
I've googled to death too... The shape of the mass is identical in both instances so must be specific to something!! I've sent the pictures off to a couple of places so hopefully someone will come back with an idea...
I'm pretty sure they're not alderfly eggs
Be sure to let us know if you get positive ID!
Can't believe that I've only just realised that your pic looks more like a mound of larvae, not eggs ... you could detach a few & put them in a pot with iris leaves to feed on to see how they develop?
Have you considered Iris sawfly? www.rhs.org.uk/.../profile