Dave commented:
Jack, I could be wrong but that plant does look like cuckoo flower, if its not, must be related to it.
Definitely not Cuckoo Flower with which I am familiar. The picture is perhaps confusing in that it suggests the flowers have a touch of mauve (as does Cuckoo flower) but they were in fact more or less white. Moreover, it was a much taller plant being about 60 cms tall. Anyway, it was identified by Dr. Martin George, past Chairman of The Broads Society and he certainly knows a thing or two. Horse Radish is indeed distantly related to Cuckoo Flowers (as it is to Hedge Mustard, etc) being a crucifer.
But I have to admit that while taking the photos, I could hear a Cuckoo distantly across the Fen - sadly a somewhat unusual sound nowadays. But I don't think that's what you meant Dave
Your unidentified tree must be an elm of some sort.
Jack