Answers found......
Thank you one and all for your wise counsel.
I've managed to find out more, as follows:
Aircraft were Dornier 217E-4 of KG2, with some Heinkel 111H of KG100 acting as pathfinders, operating from airfields in the Netherlands. They, it seems, had been brought back from the Eastern Front for the Baedekker Raids purely for pathfinding duties.
Total number of aircraft is as yet unknown but the cathedral city raids were typically 40-70 aircraft.
Bombs dropped were a mixture of high explosive and incendiary in the ratio of 1:2 by weight. The HE was "mostly" 500kg, with "some" SC1000RDX 1tonne bombs. Given the lack of other damage in the cathedral precincts I must suspect that the bomb seen just missing the cathedral tower was a 1,000kg. The Dornier 217E-4 could carry 2 x 1,000kg, so it is possible that the bomber that hit the library also dropped the bomb that nearly hit the tower. A lot of the HE landed very close to the cathedral, with 15 craters in an area enclosed by a circle of 100 yards radius.
I cannot find any reference to anything larger than 1,000kg being used, nor any trace in the National Archives of a report on bomb debris recovered from the cathedral precincts, and conclude that it is a myth.
The raid lasted two hours, leaving 43 dead, 48 seriously injured, and 50 with slight wounds.
Given the location of the epicentre and the speed/altitude/course of the bombers, I'd have to estimate that the average error is aiming was pressing the release button no more than half a second late.
Main source: The Blitz, then and now, Volume 3, edited by Ramsey, published by Battle of Britain Prints International, 1990.