Aloysius535 is correct in stating that police investigations which do not result in a conviction or caution will not show up on a CRB check at standard level.
However, if an enhanced level CRB check is carried out (e.g. because the subject is applying for employment working with, or alongside, children or vulnerable adults), senior police officers (and others agencies, such as the Department for Children, Schools and Families) can include any information which they deem to be relevant. (Indeed, following the Soham murders, they're actively encouraged to do so). Such information can include unproven allegations made against the subject of the check.
Chris