Disclosures are primarily designed to be used by an employer at the point of recruitment for a particular position. Ultimately it is for each employer, and not the Criminal Records Bureau (CRB), to decide whether a new disclosure should be applied for, bearing in mind their legal and other responsibilities and subject to any statutory requirements.
There are a number of reasons why an employer may not wish to accept a disclosure that has been processed for a previous employment position, including:
The disclosure may not be at the right level (there are two different levels of CRB check; standard and enhanced);
Information revealed through a CRB check reflects the information that was available at the time of its issue—a person may have committed a crime in the intervening period;
The disclosure process may also include a search to establish whether an individual is subject to a direction under Section 142 of the Education Act 2002, or a check against the Protection of Children Act and Protection of Vulnerable Adults PoCA and PoVA) lists.
The CRB does not hold data to show the number of people who are required to apply for a further clearance after changing jobs.