Sounds like it could be the big one. It can grow up to 5m tall with leaves up to 1m long. Sometimes red spotted stem. It does like dampish areas. It grows in some parts of Norfolk.
Warning it is a nasty beasty!! Do not get any of the sap on your hands.
On a recent walk fairly close to home in Norfolk we went through an area on a well walked footpath. At intervals English Nature had erected warning notices with an illustration, only problem was that some of the illustration showed palm trees. Now this I find quite worrying as they are supposed to be the leading light on all things natural in England. If they cannot attach the correct illustration of a plant to a warning notice what hope is there!
Glad to see that you are wearing gloves. I understand the sap causes blisters! They are impressive though.
Whenever I see them I sing the old Genesis song to myself. It is about the Victorian explorer who went off in search of the Giant Hogweed to bring it back to the UK. I wonder if anyone has wrote a song about Grey Squirrels?