Here's a photo of the Leaf Celery plant flowering (It got even bigger later). Notice the black container at the right of the photo, with asparagus growing in it.
Here is that black container now - with thousands of tiny Leaf Celery seedlings coming up in it!
In amongst the shingle underneath there are also loads of little seedlings.
I'm not going to be short of Leaf Celery for a while! I suspect that many of the seedlings will not survive the forthcoming Winter, but equally, some will. I just wish that Parsley would grow this well for me too.