This case was reported incidentally in the book Reflections on the Lakes by John Wyatt (1980), a collection of autobiographical tales relating to the author's travels around the Lake District. Surprisingly, the subject of UFOs comes up in the chapter "Apparitions and Hallucinations", and with it, the report of a circle in rushes.
On page 171 (right), we read,
“In the
centre of the very large rush bed that had been the tarn was a quite distinct
completely circular area of flattened rush. It was I suppose some fifty feet or
more in diameter. We stood there and looked at it and laughed. There was no
explanation we could readily think of … we searched on over the hill towards
Easedale Tarn. But as we rested and looked back from the higher position, we
saw the circular area quite distinctly.”
The date of the sighting is not given, but according to Crop Watcher magazine (issue 14, page 5) the circle appeared circa 1968. (We would like to confirm this at some point.)
The text mentions the "tarn", which is a mountain lake. The location, Blindtarn Moss is a semi-dried tarn in Cumbria, in the area west of Grasmere Lake.
Below is a map of the general locale, plus an OS map on which Blindtarn Moss is labelled, and an aerial photograph with Blindtarn Moss outlined in black.