As defined by the Rehabilitation of Offenders Act 1974 and subsequent regulations, under section 4.2 you do not need to declare convictions which are 'spent'.
However, by virtue of the Rehabilitation of Offenders (Exceptions/Amendments) Order 1986 those provisions do not apply if you are applying for a job supervising, caring for or otherwise connected with people from the following list, you must always declare any convictions and/or cautions for criminal offences, even where they are'spent'. For these purposes, this includes working with children, young and older people, those who are dependent on alcohol or drugs, and those with mental or physical disabilities, illness,injury or deformity, including people who are blind, deaf or without speech.