looloo you make a very important point, CRB certificates do not guarantee anything. MTTM is right if a person is working with children or vulnerable adults their employer needs to obtain an enhanced CRB. Even if the certificate says "nothing recorded" any investigations or concerns will be disclosed to the person requesting the check by the chief constable in a separate letter.
Under this system Ian Huntley would have been prevented from taking up his job at the school.
The reason you need a CRB for every job or volunteer position is that any additional information is covered by Data protection Act and cannot be shared with another party. So if I sack a new coach because of what I get from a CRB it is against the law to tell the rugby club down the road if he turns up there next. That's the theory at least; in practice we would share information discreetly.
A new agency is in the process of being set up in the UK which will hold a list of every person excluded from working with children or vulnerable adults and which can be accessed by all organisations.
I am secretary of a junior football club with around 200 members and 25 volunteers. The Football Association has blocked many people from the game as a result of stringent CRB process but its only part of the picture.
We interview new volunteers, take up references, send them on child welfare courses and mentor them. Other clubs get a false sense of security because a coach can wave a clean CRB certificate in their face.
This is the Welfare section of our club website that outlines how we try to keep the children safe. Constant vigilance is the key, not just a desire to tick a few boxes.
Bottom line is exactly as you said, do not trust somebody just because they have a CRB
Another interesting question would be whether all UK elders ought to be getting CRB checks?