A fertile shoot of horsetail, Equisetum arvense.
I took a specimen home with me. This is a close-up of the sporangiophores:
It yielded spherical spores of about 50x50um (magnification x600, field width 86um):
Some seemed to be entangled with some tubular structures:
First photo and specimen taken in Whiteknights Park, Reading, UK, on 2014-04-06.