Male catkins of hazel (Corylus avellana).
I took the following specimen back home with me:
This yielded the following pollen (magnification x600, field width 86um):
This and other photos I took show two distinct sizes of pollen grain. The larger ones are about 31 x 28um and the smaller about 24 x 21um. I am not sure why there are two sizes.
First photo and specimen taken in Whiteknights Park, Reading, UK, on 2014-01-11.