any chance of a picture of the flower? that might help with the id. :o)

Datura?

Thanks to you both, although this plant is almost 5 ft tall I am now certain that Datura in the plant in question. A nasty plant!!! The axe is coming out!!!! and a fire prepared.

The idea that it may come from bird seed is interesting!!! Strange that it is supposed to be relatively common and yet I have never heard of it - such a big, strong, tree of a plant!!

I once thought I had sunflowers coming up in my side border only to find out they were Jerusalem Artichokes from next door, The leaves a very similar but I don't think they grew that big, and I did not allow them to flower, sorry.