[MalcW]

When I took on and cleared our plot last year some of these appeared in one of the beds. I didn't know what they were, and others weren't sure, so I let them grow. Now they look like this, with the tallest ones being around 4ft tall.

[Baldy]

Maybe Egyptian / Walking Onion

http://www.provident-living-today.com/Egyptian-Walking-Onion.html

I'm growing some myself though they aren't as advanced as yours (if that is what they are) so I'm not 100% sure

Cheers,
Balders

[ptarmigan]

My guess is an alium of some kind. My guess would be a left over leek (or possibly onion) from last year pushing up a flower head and about to go to seed. You could leave it in for the bees - and the seed head for the birds. If you have space.

[mumofstig]

It's an onion rather than a leek (different kind of leaves I think)

[ptarmigan]

Oh yes, on second look - not a leek at all!

[MalcW]

OK, thanks all. So, assuming it's an onion (and not a triffid), should I be able to save some of the seed for next year? And if so, is there anything special I need to do to store them?

[beesrus]

I'm new to growing onions from seed this year. There's a lot of effort involved compared with sets and a long growing period. Personally, I don't think I would put that effort in and dedicate space without knowing what the seed was.  Once my Ailsa Craig have done their thing this year, I will however be saving a couple of the best to let them flower for future seed.
I'm at present experimenting with some unknown sweet pepper seeds and they aren't looking too good at all compared to the other known pepper seed I am using this year, probably a waste of time. One never knows if these seeds are FI/hybrid sort of thing that can go a bit pear shaped.