Dear Mr Mac,
Thanks for the reply.
Id read about the 4000, 8000, 12000Lb 'cookies' which were called block busters as they were meant to clear blocks - as you say by blowing off rooves so that incendiaries can call into the more flammable insides of buildings. I think the sizes were simply created by bolting 4000Lb-ers together. Also I recall reading that they started off with one 'pistol' but to increase the % that went off this was increased to three pistols. Which kind of ties in with what you are recalling too.
I'm wondering if the failure rate also was due to the various delayed action fuzes that were used that were set for various time delays to impact on repairs. Slave labour was apparently used by the German UXB crews.