Following the Soham murders, police are now expected to ensure that all unsubstantiated allegations against a person are always shown on an enhanced CRB check. (Ian Huntley had faced several allegations of sexual misconduct but, because nothing was proven, that information didn't appear on his enhanced CRB check).
The police will only delete such information from their systems (or leave it out of their submissions for an enhanced CRB check) when they're 100% convinced that the allegations were wholly false. Since it's extremely hard to 'prove a negative' that means that the vast majority of such allegations will always appear on enhanced CRB checks.
There has been a post, here on AB, whose CRB check showed that an allegation of grooming a child for sex had been made against him. At no time had he been questioned about that allegation, or even been made aware of it. So it seems likely that the police were of the opinion that the allegation was false (or certainly that it was unprovable) but they still exercised maximum caution and included the allegation within the check.
Chris