Triffid? Jack's magic beanstalk? or what?

Gardening people - can you help, please?
When this plant first appeared in my bird feeder base my friend said 'that's ground elder, you want to get rid of that!' Well, it couldn't spread anywhere as the base is a half barrel with no actual soil in it, just rubble, and the rest of the garden is paved, so I left it. 18 months on and this is it. I'm not worried about it, the birds love it - it gives them somewhere to perch and shelter.
But, what is it?  According to 'Google' Ground Elder only grows to less than a metre high and is often confused with Elder - this is a bit over a metre from the top of the barrel, and has thick woody looking stems at the base. So is it actually Elder? The flowers are similar but not sure about the leaves. There is Elder growing in the hedgerow round the edge of the field so obviously birds eating the berries then pooing while using the feeders could have planted it. Will autumn prove it one way or the other? do they both have the berries after the flowers?  Close up in the Extra.