Hemlock Water Dropwort*, which just happens to be the most poisonous native British plant.
So don't eat it; don't pick it; don't even touch it if you should see it.
I don't think I've seen any along the towpath, but it is growing in profusion around the winding hole. I will keep an eye on it and post photos as it progresses throughout the year.
*Thanks to Izzy for the link.