Mystery Oil Spill in Greater London. Swans Rescued.

Well this is an odd one. Where might this type of thing have come from?

At least 80 swans have been removed from the River Thames as part of a rescue operation following an oil spill that continues to baffle authorities. Thames Water responded to calls at 10:30pm on Friday night to the stretch of river between Windsor Bridge and Eton Bridge. Around 20 volunteers are working to remove the swans from the water and so far approximately 80 have been lifted out. Rescued swans were being pulled from the black water covered in oil reaching all the way up to their necks.

(Source: RSOE Disaster Index)

I wouldn't have thought that a likely place for such a thing to happen, particularly as an unknown mystery? Any locals have some insight?

I feel bad for the fishes though or any other animals down there.