Too right Tony, the Don was there before industry, and will still be there when industry (and oil!) is no more, which may be sooner than we think!
DB, I know your not having a pop.
When evrr i see oil on the don I do ring the hotline (admittedly not every time-only if it is worse than "usual"), But what can the EA do?, they can't go door to door asking "excuse me, but you hav'nt chucked a barrel of oil down the drain by any chance have you?"
Ron is right, what is needed is some form of barrier on drains that flow directly into rivers.