Clark wrote in
Could it be an explanation for the so-called
aurora sighting in the north sea thats often cited on documentaries.

No. The D-21 was canceled after only a few test flights.

http://www.amarcexperience.com/AMARCArticleD21Drone.asp





Read to the end of the article, 4 D-21D models were launched
from B-52 motherships and overflew China to the Nuclear test
area at Lop Nor.
Ben Rich in his book about the Skunk Works detailed his
getting a "Gift" from a CIA agent, it proved to be an hatch cover
from a D-21B that had flown off course in China, the hatch was
reportedly picked up in Siberia, Russia

its unlikley that they were ever over the North Sea.