Cat wrote:
[...]
There was seeds
everywhere, including on my sweater.

That's what I was afriad of.

Just a couple of questions: is it a weed, or not? It actually
proved
quite a handsome plant, if one a bit devoid of colour.

That's one of those "how long is a piece of string?" questions. If a
plant is where you don't want it, it's a weed. It doesn't really go
like hell except in cultivated ground, so if it pops up in a wild
patch, it isn't a problem. But in your tilled beds, it's not
decorative, endangered, or of any great kitchen value, and you want
your work to be for the benefit of things you like; so knock it out.

But has it any
other use? Medicinal? Culinary?

I don't know of any medicinal uses, but you can cook it as a green
vegetable. If you eat it in quantity day in, day out, it will make
you ill -- one of the symptioms is jaundicing of the skin -- but not
incurably. I'd use it a couple of times in a year if I had no other
green, but I've never tried it.

And how did a single specimen end up in my garden - bearing in mind
the gazillions of seeds it produces?

You've answered your own question, really. It produces a lot of seed,
and it seems they may survive for many years in the soil until the
conditions are right -- i.e., you come along and cultivate. It
doesn't compete too well with other plants in the wild, which is
often true of plants which produce huge quantitiies of seed. In those
ideal conditions, all it needs is one seed to come in stuck to your
boot if there aren't any in the soil already. I got some lovely
evening primroses that way, but usually what turns up is a waste of
space, like this.

--
Mike.