Whether you need to declare the conviction depends upon three things:
1. Your age when convicted.
2. The sentence you received; and
3. The nature of the job you're applying for.
The Rehabilitation of Offenders Act states that, after a fixed period of time, most convictions can be regarded as 'spent' when applying for most types of employment. This period of time is 5 years for non-custodial sentences, 7 years for sentences of imprisonment not exceeding 6 months and 10 years for other prison sentences up to 2.5 years. (For longer prison sentences, convictions never become spent).
Those periods are halved for people who were under 18 years old at the date of conviction.
Some job applications are exempt from the provisions of the Rehabilitation of Offenders Act. i.e. convictions are never regarded as spent when applying for these jobs. These are primarily jobs working in, or alongside, the 'caring professions'. (e.g. teachers, school caretakers, doctors, nurses, hospital porters, care home staff, etc). Jobs involving the administration of justice (e.g. police or court staff) and jobs involving national security (e.g. employment at a nuclear power station) are also exempt from the provisions of the Act. Where such exemptions apply, this must be clearly stated on the application form.
If your conviction is 'spent' (and your application for employment is not within one of the exempted categories), you can lawfully answer 'No' if you're asked if you've ever been convicted of a criminal offence.
Chris