Well, I have heard this story a zillion times, and it is in a lot of History books (and even on the town of Coventry’s web page!), but there is a good deal of doubt about the veracity of this story AS COMMONLY TOLD.
A conflict the size of WWII generates an enormous amount of military radio traffic, and sorting out time-sensitive operational orders from other traffic is not an easy task. By the time they had the exact details of the raid figured out, it was a bit late to evacuate cities or shuffle forces around. Instead they relied on electronic counter-measures (which failed) and the normal fighter intercepts (which failed).
From the Oxford Companion to World War II, edited by I.C.B. Dear & M.R.D. Foot (Oxford U. P. 1995)'s entry on Coventry (p. 275)
". . .attacked on the night of 14/15 November 1940 by German bombers employing for the first time, their Pathfinder force KG100, and the X-Geraet beam system for finding their targets.
ULTRA intelligence and prisoner of war information forewarned the British of a major Luftwaffe operation (Moonlight Sonata) against a number of their cities. These included Birmingham, Coventry, and Wolverhampton, but the information was not correlated and there were also indications that the targets might be in London and the south of England. In any case, it was impossible to tell which would be attacked first and the ENIGMA signals giving their direction to the stations emitting them were not broken in time. By 1500 on the day of the raid the beams were found to intersect over Coventry, but electronic counter-measures failed to work as the jammers were incorrectly set. The fact that Coventry was to be the target that night was passed on to RAF Fighter Command but 'British counter-measures proved ineffective: of the 509 bombers the German Air Force dispatched to Coventry, 449 reached the target and only one was certainly destroyed (F. H. Hinsley, _British Intelligence in the Second World War, Vol 1., London, 1979, p. 318). This failure probably hasten the departure of Dowding from Fighter Command and from it grew the myth that Coventry was left to the mercy of the Luftwaffe in order to protect the secret of ULTRA"
Also see Ronald Lewin in “Ultra at War” (Pocket
Books paperback edition, pp. 98-103).
“The Ultra intercepts did not state that Coventry was to be bombed; they specified a target by a coded designation which the British could not interpret; they in fact believed the attack would be on London or the southeast.
Also note DeWeerd, Harvey A. “Churchill, Coventry and Ultra.” Aerospace Historian 27 (Dec. 1980): 227-229
-jack