The process for applying for your CRB is to request all data held under the Freedom of Information Act 2000.
Usually a section 40 disclosure request will be sent to you from Scotland yard and if you satisy the requirement to ID a fee of �10 is payable and you receive all convictions etc.
The process is very long and 2 - 4 months is not unusual.
In West Midlands police all detained persons are fingerprinted, DNA tested and photograhed as part of the documentation process.
This is completed before the investigation ( interview ) and is now rarely questioned.
If the person is found to be innocent then the normal disposal will be No Further Action ( NFA ) , however if they are bailed to return to the police station and then Insufficient Evidence is deemed to be available to charge the record will be closed as such.
The PNC record of that person will show the arrest, the bail, and the disposal or result if charged and sent to court.
When applying for your CRB check this will only show cautions and convictions, the above police process is only recorded on PNC and forms part of the individuals police record not criminal record.
The process means that more people have thier DNA on record which is used in speculative searches for both outstanding and further crimes.
We recently had someone wrongly accused of rape and he was subjected to the documentation process, although he was completely innocent he now has his DNA photograph and fingerprints on record and is also recorded on PNC as arrested and bailed for rape then insufficient evidence.
This caused great debate but senior management and the police authority refused to dispose of his details and details of his arrest were disclosed in a CRB check in error.