The Last Wall — David Annandale

Despite the Imperium's best attempts to forestall the ork plague that is wrecking havoc in human space, an ork attack moon now hangs over Terra. 

Despite the Imperium's best attempts to forestall the ork plague that is wrecking havoc in human space, an ork attack moon now hangs over Terra. As its malevolent presence gazes down at the Imperial Palace, terrified citizens run riot in the streets. In a last-ditch attempt to destroy the attack moon, a proletarian crusade is launched. Thousands of ships large and small head to the moon carrying billions of citizens, all eager to take the fight to the enemy. It seems the attack has a chance of success as the invasion force lands safely, but the orks have yet to spring their trap...