I think Peter we was correct that NATS is simply implementing a pre-determined course of action for such an event, but I doubt they imagined that the closure of controlleed airspace would continue this long or cause such an incredible cost and disruption to the struggling aviation industry. It is fairly obvious that this catastrophe simply cannot be allowed to continue indefinitely. Someone will have to take the bit and make a practical decision. A similar event occured in the nineties when the Vatnajokull volcano erupted in Iceland. Aircraft were simply rerouted around the area with no resulting incidents and this was some ten years after the Eric Moody episode! IMHO in the weeks and months ahead this whole episode will prove to be an understandable overeaction. Yes ash is damaging to A/C and engines but only in v high concentrations such as were experienced by BA009.
NATS have admitted they dont even know what the concentrations are and yet they have made this kneejerk reaction which I cant help feeling is down to an element of paranoia concerning aviation ever since 9/11.
In future I doubt if airspace will be closed in this fashion for similar events. MET will issue the warnings and it will be down to individual airlines to decide how to avoid the danger. After all it is their A/C that are on the line and no airline is going to deliberately risk passengers or A/C in a foolhardy flight which could ruin their reputation and be financially disastrous. But why should they suffer financial ruin through no fault of their own because of an overcautious bureaucratic ban?