There are further problems with these checks. They rely on information held on the police national computer, unfortunately some of the info is not up to date so if you were convicted of assault last week and it hasn't been input yet, so you get your pass.
Some people with common names get offences atributed to them they never did, a example of this occured in a town I worked in a few years ago, a woman was convicted of shplifting and assault on a police officer - she lived in Kings Terrace, another woman living in Kings Road with the same name applied for a job in the local Police station a few months later and was turned down as she failed to disclose her criminal record!
Also I understand that some older convictions were held on manual systems and were never entered in the PNC! so if you mugged an old granny , or dealt a little smack in the 60's it may not show up!