Charlie Higson 
The Enemy Series, Books 4-6 

They»ll chase you. They»ll rip you open. They»ll feed on you . . .
The Sacrifice (Book 4)
Small Sam and his unlikely ally, The Kid, have survived. They»re safe with Ed and his friends at the Tower of London, but Sam is desperate to find his sister.
Their search for Ella means Sam and The Kid must cross the forbidden zone. And what awaits them there is more terrifying than any of the horror they»ve suffered so far . . .
The Fallen (Book 5)
The Holloway crew are survivors. They»ve fought their way across London and made it to the Natural History Museum alive – just. But the fight will never end while the Enemy lives . . .
The kids at the museum are looking for a cure. All they need are medical supplies. To get them means a journey down unknown roads. Suddenly it»s not so clear who – or what – they»re fighting.
The Hunted (Book 6)
The others had promised that the countryside would be safer than the city. They were wrong. Now Ella»s all-alone except for her silent rescuer, Scarface – and she»s not even sure if he»s a kid or a grown-up.
Back in London, Ed»s determined to find her. But getting out of town»s never been more dangerous- because coming in the other direction is every SICKO in the country.