The Bare Bum Gang and the Holy Grail by Anthony McGowan
Ludo, Noah, Jamie, Phillip and Jennifer are THE BARE BUM GANG! They have an embarrassing name but a cool Gang Den, so things could be worse. Join the gang in a new adventure as they battle the forces of the evil Dockery Gang, guard with their lives the secrets of their underground sweet stash, and still get home in time for tea.
Ludo and Noah go to the aid of an old tramp who has collapsed after being hassled by the Dockery gang. Everyone thinks he's a little bit crazy as he dresses in home made armour - his nickname is King Arthur. King Arthur asks Ludo to retrieve his 'treasure' and so the gang have to sneak past security guards and overcome various obstacles to help the old man. But can they get to the treasure before anyone else?