However, people who are specifically asked if they have cautions, reprimands or final warnings should disclose them until they are deleted from police records. Records of cautions should be deleted after five years if there are no convictions on the record. (In practice, some police forces may retain records of cautions for much longer than this or indefinitely.)"
Source:
http://www.lawontheweb.co.uk/rehabact.htm
The CRB website states that their discloures show "details of all convictions, cautions, reprimands and warnings held on the Police National Computer" but "Ex-offenders will retain the protection afforded by the Rehabilitation of Offenders Act 1974". However, it doesn't make clear their policy on cautions (which, as I've said, aren't covered by the Act):
http://www.crb.gov.uk/Default.aspx?page=1871
(It would seem reasonable to assume that cautions are treated in the same way as most non-custodial sentences. i.e: they will become 'spent' after a period of 5 years. That also seems to be consistent with the NACRO statement).
For an authoritative answer, phone the CRB on 0870 90 90 811.
Chris