Relevant information will be posted on the Eurocontrol NOP site - see top right, although if that's supposed to be a link it doesn't open for me.
https://www.cfmu.eurocontrol.int/PUB...pec/index.html
Interestingly, information should be diffused through the VAAC (Volcanic Ash Advisory Centre) system which caused so many headaches last April, when the London VAAC was being over-zealous. It seems to be the opposite case with the Tokyo VAAC this time around.