Under the old Criminal Records Bureau system people who wanted to work with children had to gain clearance for each group of children they worked with. Not only was this inefficient but it was always out of date because it was a one-off criminal records check - it took no account of criminal activity recorded after the check had been carried-out.
The new DBS check allows individuals to undergo a single clearance procedure which stays with them for life if the individual registers for the government's DBS Update Service (here) because their clearance is then checked and updated weekly and the results are always available online. This allows organisations to instantly verify each vistor's DBS clearance.
To perform an online DBS check anyone can simply…
Organisation name, your forename, your surname.
DBS Certificate Number, Surname, Date of Birth.
… to receive a simple statement (which can of course be printed and retained for your records) about the individual's live DBS status.