Help Needed: Mystery Plant ID Please!

Shirl over at Shirl's Gardenwatch just put up this drool-worthy post about a new Piet Oudolf garden that she discovered this summer at Pensthorpe, in the UK. If you're already a Piet fan, it's a must-see post... and if you're not already enamored of his work, you might be once you check out this new garden!

A mystery plant in one of the photos Shirl posted really caught my eye. Dark foliage, pretty form, great texture... it's not much of a surprise that I had to ask Shirl about it. Seeing the small-sized picture, I guessed that it might be a young, dark-leaf barberry. Shirl wasn't sure what it was, either, but didn't think it was a barberry. She was nice enough to send me her original picture of this area, though, so I could zoom in better on the plants in question... and I agree with her that it probably isn't a barberry.

Anyone want to take a crack at an ID for us? I uploaded the large-size original photo that Shirl emailed me, so you can click it to see the plants in better detail. You can see them behind the sedums in this picture:

We both agree that it probably isn't a barberry (berberis) but haven't hit upon a positive ID yet. My best guess is black cow parsley (anthriscus sylvestris 'Ravenswing') but that's just a guess. I get the impression that this plant is much more fine-textured even than the cow parsley... but I have never seen that growing in person, so I really have no idea.

Shirl and I both really like this mystery plant, and of course feel like we "need" to add it to our respective gardens now that it's come to our attention, so any and all attempts at an ID will be much appreciated! Even if you can "only" help us rule out some of the possibilities, that would be great... :)