Hi Ben,
There's Reading Roadrunners (google them, I don't remember the url right now), which is a big club and organizes several local events. They meet at Palmer Park (or did when I was a member years ago).
There's also Reading Joggers, who I think have a web page. They're a smaller and, dare I say it, less "serious" outfit (I hope they won't mind me saying that!)
Also, the guys at Sweatshop do runs every Monday evening from the shop, opposite the railway and bus station if you're after something less formal.
Hope this helps!