Thanks PP and Tocsin (now I know where Tocsin comes from). The Royal Observer Corps of WWII is legendary, had no idea they had a mission into the nuke age. Wikipedia has a lot of info on the ROC and their equipment, including the Bomb Power Indicator and Ground Zero Indicator. Most interesting, much more organized and thought out than my experience. I'm thankful that the balloon never went up, though the incidents in this thread are hair-raising
The UK Monitoring and Warning Organisation of which the ROC was a part made a couple of films that showed how the organisation was to have worked in a case of a nuclear attack. Some of them are now on youtube. The 1962 one has some good footage of RAF air defences going into action to boot.