Section 31 of the Police and Criminal Evidence Act:

  Where—

    (a) a person—

      (i) has been arrested for an offence; and

      (ii) is at a police station in consequence of that arrest; and

    (b) it appears to a constable that, if he were released from that arrest, he would be liable to arrest for some other offence,

  he shall be arrested for that other offence.