Is the job subject to the rehabilitation of offenders act, or is it exempt? if it is exempt it will say "this post is exempt from the Rehabilitation of Offenders Act 1974" on the application form, in this case you must declare all offences whether spent or not, if it is not exempt any spent convictions, such as cautions, which are spent immediately, need not be declared and they cannot legally take them into account if they show up on the CRB.