[bobbyt]

Hmmm.......not sure, I've never seen one in flower, assumed they didn't, maybe should let one grow fully and see what happens so there can be a positive id.

[greenjay]

 Hi
It's a type of rosebay. I forget it's proper name. Pops up regularly on my patch. If you leave it seed you will definitely see it's a type of rosebay willow herb. Feathery seed heads.

[Alank]

I also get them on my plot they blow in from other unattended plots.
They are an absolute nightmare and can put down roots anywhere sometimes forming a matt of tiny green plants early in the year.
When they mature they resemble a very small willow herb but instead of the long stem with the pinkish flowers these have side stems also and produce a profusion of floating seed the same way as the willow herb.
I agree with New Shoot that the only way I've found to control them is hand weeding.
I would not let them take over an area because then you would have to deal with the roots.
I've had allotments for a long time in the same area and have only noticed these over the last five years but they are everywhere on our site now.
Good luck 

[bobbyt]

They are a nightmare as you said, seem to be a lot more now than previously. I have a 3 x 2 M patch which I covered with plastic to clear the couch grass with patches of these under there, will this kill them off?

[Alank]

Covering with black plastic should do it.
If they are the same as the ones on my plot if you cover them before they a mature plant then they should die off.

[Alank]

As you can see from the old stem they don't grow as tall as willow herb but they are just as troublesome 

[Richard Yates]

Parsnip like leaves and orange roots.

Well, sort-of parsnip-like if you have a squint. Orange roots, often forked. Would not be surprised if it's poisonous/inedible.

It's on what looks like a piece of bread, but that's in fact a brick weighing down my compost bin lid.

What? No! BRICK!