without knowing the size of the plot I can give some suggestions.
hosta, ferns, ligularia, gunnera, skunk cabbage. candelabra primulas to name a few.

I notice you are in Norfolk if you can visit Fairhaven gardens near Wroxham/Ludlow. Check for location as I was on holiday in this area and visited it. They have a fantastic water garden and you will have excellent ideas from them.