Not all convictions are 'spent' after 5 years. The length of the 'rehabilitation period' depends upon the sentence passed by the court:
http://www.lawontheweb.co.uk/rehabact.htm
(As you're probably aware, no conviction is ever regarded as spent when applying for certain types of employment, which are exempted from the provisions of the Rehabilitation of Offenders Act, such as those which involve contact with children or vulnerable adults).
At present there are two levels of CRB checks. ('Standard' level checks only show convictions or cautions. 'Enhanced' checks can show any other information which a senior police officer, or other relevant person, deems to be relevant. These can include non-substantiated allegations or even information about the person's partner or family members).
With both the present levels of checks, all convictions will
always show up. However the CRB is currently working upon the introduction of a 'basic' level check. There's very little information available at the moment but the assumption seems to be that this will become the 'normal' check for 'non-sensitive' occupations, with spent convictions not appearing on it.
Chris