I have an update on my monster weed. It reached around 8 or 9 feet and then perished in the frost the other night. I cut its branches off and dug it up the other day, so can show you a picture of its root system - very shallow considering its size.
It had begun to develop flower buds, but these never opened. I did noticed however, that what seemed to be the same plant, growing in a container I used to cause my radishes to bolt, had developed a flower several weeks ago. I am not sure this is the same plant, but it has the same shape to its leaves, albeit in miniature, and the same colour and pattern to the stem - red at the bottom spreading upwards. I had hoped it would set seed, but it too was killed in the frost, and the flower head I rescued has thin beige husks within, which aren't hugely helpful I fear.
To add further confusion to the mystery, another monster weed developed alongside the original, and also on the other side of the house. It seemed alike in every way, except it had very furry leaves - you could really feel the bristles catch on your skin when you touched it. It developed several tiny flower buds on each branch, that were due to be yellow I'm sure, but they also were frosted before opening.
Here are the photos - any ideas what this intruder could be?