Most likely Heracleum mantegazzianum, giant hogweed.

Maybe Stinging Nettles. There are always Docks where there are Nettles. Nature provides the cure. Rub a Dock leaf on to the stung area, its most interesting how it soothes the stung area.
I dont know if it would work with Hogweed.

if it was giant hogweed you should wash the blister and keep out of sun, can be very serious otherwise.

Sounds like giant hogweed to me. Vitamin E oil helped when our son tried to tackle this one summer without wearing gloves. Docks don't work for this one and I'm not really sure they work with nettles it is the rubbing action that soothes the 'sting'

dock works by folding the leaf several times to release the juice then rub on and it instantly soothes it :)

Docks don't work on giant hogweed and never seemed to work for me when I was little and always falling in the nettles!

Agree with Cammomile there is no evidence to suggest that docks stop the sting from stinging nettles - I would think aloa vera would work a lot better for nettles, doubt it will work for giant hogweed though. When son decided to try to cut back the beast he and his friend were both burnt - yes the sap actually burns- hence my use of vitamin E oil. Parent of other child didn't use and he ended up with scars.

when i was caught out by hogweed i got a gauze and covered it with sudo cream, secured to my blister and went within a week. but was told to keep out of sunlight because this can cause serious reaction and you could end up in hospital! but as moon growe said basically treat it like a burn.