From the OP:
no file downloads, no porn, no webmail etc. We monitor usage in co-ordination with blocking software
I would never work for such a company. It blows my mind how anyone would. It must be that because of all the click-through EULAs, we're conditioned to dismiss any legal stipulations and just figure, "Eh, it'll probably never happen to me." And largely, that is true. (I'm speaking mostly of Americans when I reference this dismissive attitude.)
I see all the time driving 70 MPH on the 55 MPH-speed limit Interstate, or driving through red lights. Who watches the watchers? Oh yeah, that would be nobody. Oh, don't worry though, I'm sure they're browsing "responsibly" and don't need watching.
Well, I do work as a sysadmin, so naturally I fall into that category. But I sure as hell haven't ever policed anyone for checking their Gmail, for Christ's sake. The only time I've ever had to confront anyone about porn usage was in a computer lab, so it was not staff perpetrating. What a staff member does in their office is none of my damn business. It's up to their supervisor to make sure that they remain productive, and I doubt anyone cares if they eat peanut butter sandwiches, or listen to Wagner, or watch goat sex on YouTube while they maintain their output.