From the other side of things - when I was in the taxi (full 'enhanced' CRB check btw) one of my regular customers worked at a home for young people and adults with special needs.
Two 'clients' were very friendly with each other so as my customer was on duty they decided to lock the door to one of them for the night so they didnt disturb each other or other clients.
That simple action cost them there job and they are now permanently on file as unable to work with children and vulnerable adults.
The taxi firm I worked for also had a genuine pedophile working for them and it was only when the enhanced CRB check was introduced (this is some time ago) that he was found out and removed. Again he would now be on a few lists as above.
Going further back, a prominent local policeman had abused children in his care whilst also working at a local community centre. He used to visit schools and organize day trips with absolutely no-one any the wiser to what he was until he went to trial in the mid 1990's.
The system cannot prevent the fire and it only seems to be able to put it out once its burnt the house down.
It can be very hard for victims to come forward, again, the act has happened, a CRB check isnt going to find this out until that victim eventually comes forward.