CRB Disclosures do not carry a pre-determined period of validity because a conviction or other matter could be recorded against the subject of the Disclosure at any time after it is issued.
If you are asked to apply for a Disclosure and you already have one for a previous role; ask if the organisation is willing to accept it. When making this decision the organisation will take into account the length of time that has elapsed since that Disclosure was issued; the level of Disclosure; the nature of the position for which the Disclosure was issued; and the nature of the position for which you are now applying. Ultimately, it will be the organisation�s decision whether to accept it or not.
http://www.crb.gov.uk/Default.aspx?page=1871