When I first saw these I thought that they must be small Bibionid flies as they were out at about the same time as the St Mark's flies (early May). However, the key to their identification is the enlarged first tarsi on their forelegs: they are actually Hilara sp (Diptera: Empididae), a type of dagger fly which lack that family's characteristic long proboscis.
[Note added 2011-05-02: The swollen tarsi contain silk glands which is used to wrap up prey during courtship. Only males of some Hilara species have these enlarge tarsi. For more details, see the comment by Matthias Buck here.]
Photos taken in Whiteknights Park, Reading University grounds, Reading, UK, on 2010-05-05.