Simon,
Ring Range Control on 49355000 for closures. Somtimes things change on short notice, but normally have a good idea.
At present I think there will be big closures for nearly all July for the US exercise and some in November for the Singaporeans. The US closes the whole area normally, assuming they still have enough money to come over.
Be careful, because a lot of the closures only affect certain areas so you can often work around a closure.