I doubt anyone will cover you for volcanic ash cloud now as it is now a known factor. I always use the Post Office Travel Insurance, and I buy it as soon as possible after booking my holiday as it covers you for any eventualities of not being able to go on holiday (eg broken leg, your boss is off sick and you can't take the time off, a death in the family). I bought my travel insurance in January even though my holiday wasn't until July. In this case I was covered for volcanic ash as I bought it before it all happened, but if I had bought it after all the problems I wouldn't have been covered as it was a known factor.