A tyranid host sizable enough to besiege a city [Warhammer 40k] vs Megacity 1 [Judge Dredd 2012]

A Tyranid bioship crashes into the wastelands outside Megacity 1 due to some freak spacetime accident or warp fuckery. Surviving Tyranid forces including command castes immediately make to besiege the city, can they overtake it, and then Earth itself?